If x^2 + 3 is not in brackets, and the expression is written in-line as you did, then it is not a numerator! What you wrote means x^2 + \frac{3}{4} +...
If we have f(x,y,z) and z is a function of x and y then we can write f as a function of x and y only, say f(x, y, z)= g(x,y), and, by the chain rule,...