Is your original expression supposed to be dy/dx = 3x - 4, i.e. the equationn of a straight line? If you've integrated incorrectly.
If it is supposed to be dy/dx = 3x^2 - 4, i.e. a quadratic, then yes, this gives you y = (3x^3)/3 - 4x + c. Of course you can cancel the 3s in the first term - it is just three times something divided by three.