How can I find the value of a variable to make an equation true instead of false?

So, this means we need to solve for x.

So, lets say we have 5 + 7 + x = 25

You start by getting x on its own. Subtract 5 from both sides (to keep it even)

25 - 5 = 20

Then, we're left with 7 + x = 20

Subtract 7 from each side.

20 - 7 = 13

We are then left with:

x = 13

If we plug x in, 5 + 7 + 13 = 25, then we find out that x = 13 is true.