Note the equal sign. What you do to one side, you do to the other.
Subtract 3x from both sides, and add 7 to the other
2x (-3x) - 7 (+7) = 3x (-3x) - 24 (+7)
-x = -17
Isolate the x. Divide -1 from both sides
-x/-1 = -17/-1
x = -17/-1
x = 17 is your answer
hope this helps