Initial momentum of the system = 0.
Crate acceleration along ramp = g*3/5 (component of g along ramp)
Velocity of crate (along ramp) at B given by v^2 = 2*(g*3/5)*L (L = length of ramp).
Horizontal component of crate velocity vcrate = -v*4/5 (with horizontal velocity positive to the right).
Momentum of system when crate reaches B: mcrate*vcrate + mramp*vramp = 0
See if you can take it from here.
This is cool!
OK I get 8.93 feet in one second. What do you get XV