Let the velocity of the vehicle be
V₁ = (21, 0)
Let the velocity of a raindrop be
V₂ = (a, b)
When returning, the raindrop is vertical.
Therefore a = 21 m/s
When driving north, a raindrop makes an angle of 36° with the vertical, therefore
tan(36°) = a/b = 21/b
b = 21/tan(36°) = 28.9 m/s
The speed of a raindrop is
√(a² + b²) = √(441 + 835.44) = 35.73 m/s
The angle is 90-36 = 54° with the ground.
Answer: 35.7 m/s at 54° relative to the ground.