Find the angle a projectile should be fired at

to hit a target a meters above the ground and b meters from the origin, where it is being fired from (ground level). I'm given the initial velocity, but not the angle. I can see that the problem can be solved, I just don't know how to do it. The velocity vectors are Vcos(theta) for x and -10t + Vsin(theta) for y, where t is time, theta is the angle and V is the initial velocity (given - treat as constant). I tried creating a unit tangent vector and then integrating it to find the path but the equation only got uglier. Any help at all would be greatly appreciated

using S= ut+1/2at^2

in x diretion
b= Vcos(theta)t
t=b/Vcos(theta) ----------1

in y diretion
a=( -10t + Vsin(theta))-1/2gt^2


substitute t from 1 and then subjet V