Angle of a projectile which maximizes the range? Cannot solve the equation?

R for this specific problem is R = (Vacos(theta) - Vc)*(2Vasin(theta))/g where V_a is the velocity of the arrow, which is shot off of the car V_c is the velocity of the car, which is moving in the opposite direction of the arrow So, to find the angle I need the derivative and set it equal to... show more R for this specific problem is R = (Vacos(theta) - Vc)*(2Vasin(theta))/g
where V_a is the velocity of the arrow, which is shot off of the car
V_c is the velocity of the car, which is moving in the opposite direction of the arrow

So, to find the angle I need the derivative and set it equal to 0
therefore

2V_a/g * [2V_a cos^2(theta) - V_c cos(theta) - V_a] = 0

How do I Solve it??
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