Let m1 = mass cannon+platform =46kg.
m2 = mass ball = 10kg
u1 = u2 = initial horizontal velocity of m1+m2 = 3ms^-1
v1 = final velocity of cannon and platform = -2ms^-1
Use m1u1 + m2u2 = m1v1 + m2v2 to find m2v2 the horizontal momentum of the ball after firing. Rearranging:
m2v2 = m1u1 + m2u2 - m1v1
= (46 x 3) + (10 x 3) - (46 x (-2)) = 260kgms^-1
Now let m1u1 be the same horizontal momentum when it hits the block.
m1u1 + m2u2 = m1v1 + m2v2 note that v1 = v2 which is the final horizontal velocity of ball and block combined. Rearrange for v1:
v1 = (m1u1 + m2u2)/(m1 + m2)
= 260 + 0(stationary block)/10 + (32/1.62 (mass block))
= 8.74ms^-1 this will be the horizontal component of velocity at impact at the base of the cliff.
Find vertical component of velocity at impact. Use v^2 = u^2 +2as.
v = (u^2 + 2as)^0.5 = (0 + 2(1.62)(50))^0.5 = 12.73ms^-1
The impact speed will be the magnitude of the horizontal and vertical velocity vectors added using Pythagorus' theorem:
Impact speed = ( 8.74^2 + 12.73^2)^0.5