Okay so, you would just have to simply do one step for this literal equation.
x-b=a
+b +b
x=a+b
You would just have to add b to both sides to cancel out the -b on the left side because you have to isolate x to solve for x. So you add b to both sides, and then the -b gets cancelled out and the right side becomes a+b.
So x = a+b
Hope this helped!
