An equation remains true as long as the same operations are done to both sides of the equation. The strategy is to isolate x to get it by itself on one side of the equation.
Subtract 5 from both sides of the equation.
35x + 5 = 45
- 5 -5
--------------------
35x = 40
Now divide 35 from both sides of the equation.
(35x)/35 = 40/35
x = 8/7