In any algebra problem, you just need to isolate by moving the terms around so that you can isolate the variable you want onto one side, and everything else onto the other side.
3x-4y=7
To move terms around, we just use addition/subtraction/multiplicaiton/division.
Now, we want to get rid or the -4y on the left side, we can do that by adding 4y onto both sides (onto both sides to keep the equation balanced). We get
3x-4y + 4y =7 + 4y
That simplifies to,
3x = 7 + 4y
Now, since we only want x, we can get rid of the 3 by dividing on both sides by 3 (again, both sides to keep the equation balanced).
(3x)/3 = (7 + 4y)/3
This simplifies to
x = (7 + 4y)/3
and this is your answer.