Question

-5.7=x+2.6
x=

Answer 1

First switch the sides to make it look like:
x + 2.6 = -5.7
Now you have x on the left side.

Answer 2

You want x by itself, but 2.6 is being added to x, so by subtracting 2.6 from x + 2.6, x will be by itslef.
The rule is that you need to do the same to both sides of the equation, so you must subtract 2.6 from both sides.