I am not sure how to do this question I know I have to take the derivative but then I am left w/ to unknowns.

Question

If a projectile is shot up hill the distance (range) of the landing point up the hill depends on the initial speed of the projectile,v, the angle of inclination, a, and the angle of elevation of the gun, B. The range of the projectile in metres is given by the formula
R= 2v^2/g [sin(B-a)cosB/cos^2a]
where g is the gravitational field strength, im m/s^2.
(a) Determine the value of B that maximizes the range for a hill with angle of inclination pi over 6(in radians) (30 in degrees)