Assuming you have the variable/conditional events installed...
[1] onActivate -> Self -> fireRelay
[2] onRelay -> Self -> fireRelay
[3] onRelay -> Self -> VCE_ifValue -> [ <var:client:lastmsg> ] -> = [ Cannon ] -> [ 4 4 ]
[4] onVariableTrue -> Player -> spawnProjectile -> Cannonball