The question reads:
Find the values of m for which the line y = mx - 9 is a tangent to the curve x^2 = 4y.
All I need is the basic formula? I can't find one anywhere.
differentiate both sides
now would be the gradient of the curve (m in the formula)
substitute this in along with (you are given this) into the equation of the line you want to find and solve this for x
I get
now plug these values into your expression for to find the values of m (remember dy/dx=m)