From part a)
(5m+2m−3m−3m)i+(mV−mV+2m+2m)j=0i+0j
Consider the j component. That is the j component on the left hand side must be the same as the j component on the right hand side, that is zero.
mV−mV+2m+2m=0V−2V+2+4=0−V+6=0V=6