So pretty much make your equation as 5x + 10 = 40
Subtract 10 from both sides to get 5x = 30
Then divide 5 from both sides and your answer will be x = 6.
If you want to check your answer, just plug in 6 for x:
5(6) + 10 = 40
30 + 10 = 40
40 = 40
Hope this helps!