When solving equations like these, our number one goal is to get x to be alone.
So given the equation lets go through it step by step.
3x - 5 = 10 + 7x (first step is to add 5 to both sides)
+ 5 = + 5 ( Now lets see what we have now)
3x = 15 + 7x ( We need the x to be alone! So lets subtract -7x from both sides.)
-7x = -7x ( now what do we have?)
-4x = 15 (okay divide both sides by 4 to isolate the x)
÷4 = ÷4 (now the 4's on the left side cancel out, and we solve on the right side)
x = 3.75
That's your answer :) Good Luck!