balance this equation. prove that it obeys the law of conservation of mass by using formula masses.

Question

HNO3 + Mg(OH)2 ---> HOH + Mg(NO3)2

is this balanced?
2HNO3 + Mg(OH)2 ---> HOH + Mg(NO3)2

I don't understand the formula masses part do i just find the mass of each?

It isn't balanced. Do you know how to tell if it is balanced. We simply count them up on both sides.
I see 4 H atoms on the left (2 from 2HNO3 and 2 from Mg(OH)2). I see only 2 on the right. So we can stop checking there since it isn't balanced. Have another go at balancing. For the formula masses, what the problem wants you to do is to use the molar mass (formula mass) of each, multiply by any coefficients, and add everything on the left. It will be the same mass if you add everything on the right. Let me know how come out.

2HNO3 + Mg(OH)2 ---> 3HOH + Mg(NO3)2

ive done this problem before and that's what I got with the masses equaling 208 on both sides and I got it wrong

The equation isn't balanced. I see 4 H atoms on the left and I see 6 on the right. You should be able to count them up and KNOW when it is balanced.
The balanced equation is
2HNO3 + Mg(OH)2 ==> 2HOH + Mg(NO32)2
HNO3 is 63. 63 x 2 = 126.
Mg(OH)2 = 58.3
126 + 58.3 = 184.3

On the right. HOH = 18 and 18 x 2 = 36
Mg(NO3)2 = 148.3
148.3 + 36 = 184.3
That shows you how to do it. As for your answer of 208, it CAN'T be right because if it isn't balanced then the masses won't add out. However, let's check it.
Using your numbers, the left side will be the same as above; i.e.,
126 + 58.3 = 184.3

on the right, we have 18 for water. You have 3 of them so that is 3 x 18 = 54.
Mg(NO3)2 = 148.3
148.3 + 54 = 202.3
So, it not only doesn't add up, with the 3 in front, but it doesn't add up to 208. Look this over and see where you went wrong.
You really should learn how to tell when an equation is balanced. There may be an equation I can't balance BUT I know when it is right and when I need to keep working on it because i can count up the atoms on both sides. Later on you will learn that an equation must be balanced three ways.
1. the number of atoms on each side must balance.
2. the charge on both sides must balance.
3. If it is a redox equation, it must have an equal number of electrons lost and gained.

2HNO3 + Mg(OH)2 ==> 2HOH + Mg(NO32)2
I made a typo here. Obviously the Mg(NO32)2 on the right should
be Mg(NO3)2.