Trigonometric Function Question?

I have this question and I have no idea where to start it. Any help would be extremely appreciated!

**If a ball is hit into the air at an angle of elevation between 0 and 90 degrees its path can be modelled by the equation:**

y= x (sinb / cosb ) - (gx² / 2u² cos² b)

where x and y are cartestian coordinates.

g=10m/s² (gravity), u = (initial velocity), b = the angle of projection. Lyall can hit a cricket ball at 40m/s. The boundary at the MCG is 70m from the pitch.

Is he capable of hitting a six and if so calculate the angle at which the ball should be hit to clear the boundary rope? (i.e. y=0 and x=0)

Honestly, I don't know where to start and don't know which rules to apply.

Please help :)