Define the quotient rule.
If f and g are functions of x
d/dx(f/g) = (f'g - fg')/g^2
This is just a variation of the well-loved product rule, where f/g = f * g^-1
d/dx(f * g^-1) = f'*g^-1 + f * (-1)g^-2 * g'
= f'/g - fg'/g^2
= (f'g - fg')/g^2