U = 20 m/s. Ф = 30°. To find x when y = - 40 m.
Equation of projectile: y = x tan Ф - g x² Sec² Ф / 2u²
y = -40 m when the projectile hits the ground.
y = - 40 = x /√3 - (10 * 4/3 * 1/2 * 1/20² ) x²
x² - 20√3 x - 2400 = 0
x = 10√3 + √2700 = 40 √3 meters
Range = 40 √3 m