You can put this solution on YOUR website! Math will become easier for you once you learn that if the area of a rectangle is length times width, it is always length times width no matter what the length and width are.
So if your rectangle has a length of (x+3) and a width of (3x+4) then (surprise, surprise) its area is length times width:
A = (x+3)(3x+4)
This may be an acceptable answer. Or you might want to use FOIL to to multiply this out.