Question

add the like terms together.a+b+a+2b

Okay. Well, the questoin says to add the like terms together, that means add the A's together and add the B's together.

a+b+a+2b

Alright, so your a's and your first b don't have a number in front of them. That means the coefficient is one. So it's the same thing as

1a+1b+1a+2b

Now, since its addition, you can move the order of the equation without changing the meaning. Put the a's together and put the b's together.

1a+1a+1b+2b

Now add the coefficients.

1+1=2 so 1a+1a=2a
1+2=3 so 1b+2b=3b

Now your answer would be
<b>2a+3b</b>

get it?

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