I agree...having the variable on both sides of the equals sign is confusing. SO.....let's get rid of the variable term from one side!
I see a 3x on the right side of the equals sign. If I ADD -3x to both sides, of the equation, I will not have changed the solution of the equation, but I WILL have made the equation much easier to deal with:
5x - 15 + (-3x) = 3x + 39 + (-3x)
Combine like terms to simplify each side:
2x - 15 = 39
Now, isn't that easier?? Can you take it from here? Remember to check your solution by substituting into the original equation.....