b=a+x, for a

Answer 1

Subtract x from both sides of the equation.

That will leave you with an equation for a.

Answer 2

I still don't understand

Answer 3

Is it b-x=a

Answer 4

Yes, but you might want to write it as
a = b-x
which is the same thing.

The variable you want to know the value of (a, in this case) usually is put on the left.

Answer 5

Thanks