This is a problem pertaining projectile motion wherein the parameter we're finding is the angle of inclination denoted as theta. The formula to be used is the range:
Range = v^2*sin(2*theta)*(1/g)
100 = (60^2) * sin(2*theta) * (1/32.2)
theta=31.7°
I hope I was able to help you. Have a good day!