Ashley,
I'm not sure if you mean
^{4}/_{7x} + 3 = 15 or ^{4}/_{7} x + 3 = 15
I am going to assume it is the second. ^{4}/_{7} x + 3 = 15.
You want to manipulate the equation, always maintaining an equality and end with x on the left of the equal sign and a number on the right. I would first subtract 3 from both sides. This gives
^{4}/_{7} x + 3 = 15
^{4}/_{7} x + 3  3 = 15  3
^{4}/_{7} x = 12
The difficulty now is the fraction o the left side so multiply both sides by 7.
^{4}/_{7} x = 12
7 ^{4}/_{7} x = 7 12
4x = 84
Finally divide both sides by 4
4x = 84
4x/4 = 84/4
x = 21
I hope this helps,
Penny
