Using the following formulas:
Range = [ Vo^2 * sin 2*theta ]/g
Height = [ Vo^2 * sin theta^2 ]/g
Since the relation between Range and height is given: ( R = 3H ), the 2 equations can be equated in terms of Vo^2 so that the only remaining unknown variable left will be the angle. This is done as shown:
( g*Range )/(sin 2*theta) = Vo^2
( g*Height )/(sin theta^2) = Vo^2
( g*Range )/(sin 2*theta) = ( g*Height )/(sin theta^2)
Applying Range = 3*Height:
( g*3*Height )/(sin 2*theta) = ( g*Height )/(sin theta^2)
substituting given values while Height cancels out:
( 9.8*3 )/(sin 2*theta) = ( 9.8 )/(sin theta^2)
Angle theta = 33.67 degrees
