I am not sure how to factor this out
$$f(x) = \sqrt{3-5x}$$
I then make it $f(x) = \frac {\sqrt{3-5(x+h)} - \sqrt{3-5x}}{h}$
I tried to multiply by the first time + the second term from the numerator which I called x and y
$$\frac {x - y}{h} \cdot \frac {x + y}{x+y}$$
which gives me $$\frac {x^2 - y^2}{h(x+y)}$$
From here it gets very difficult
$$\frac {5}{ h \sqrt{3-5(x+h)} - \sqrt{3-5x}}$$