a projectile is fired with velocity u at an angle a above the horizontal.. it lands on the same horizontal plane at point P. it lands the furthest point away on the plane. show that for the projectile to hit a point H above point P and its projected at the same angle the initial velocity must be u^2/root(u^2-gh)

i know that the angle of elevation is 45 degrees. im trying to use ut-.5gt^2=h