What is the best equation to use for these problems?!

4) A 225 kg cannon shoots a 3 kg cannonball. The cannonball has a speed of 20 m/s after i is shot. A) How fast does the cannon move backwards? 5) A green ball (m = 4 kg) before the collision is moving 3 m/s and after the collision is moving -2 m/s. A purple ball (m=2 kg) is moving before the collision -3.0 m/s A) What is the purple ball's final velocity? 6) A green ball (m = 5 kg) has a velocity before the collision of 2m/s and after the collision is -1 m/s. If the velocity of the purple ball (m=2 kg) before the collision is -4 m/s A) What is the purple ball's final velocity? start with mg = 4 Pi = pf P = MV Vgi = 3 Pgi + Poi = Paf + Ppf Vof = - 2 movgit mpvpi = movof + mpupf mp = 2 Vpi - -3 ( 4 . 3) + ( 2 . - 3) = ( 4. - 2)+ 2 vpf 14 = 2 Upf Vpf = ? 1 2 + ( - 6 ) = ( - 8 ) + 2 upf 2 2 +94 - 8 + 2 upf I my's = upt