Simplify:
| Fnet | = ke*q*(Qb + Qc*sqrt(2)/4)*sqrt(2)/a^2
Data:
Qb = 2.5*q
Qc = 6*q
| Fnet | = ke*q^2*(2.5 + 6*sqrt(2)/4)*sqrt(2)/a^2
Gather:
| Fnet | = (ke*q^2/a^2) *(2.5 + 6*sqrt(2)/4)*sqrt(2)
Crunch the numeric part of the expression:
(2.5 + 6*sqrt(2)/4)*sqrt(2) = 6.53553391
Answer:
| Fnet | = 6.536 * ke*q^2/a^2
You aren't supposed to plug in the values of ke, q, and a, because it tells you to leave them in symbolic form. Even though the value of ke is a universal constant. Be sure to indicate that both q and a are each squared.
45 deg CCW of +x is of course your direction. We know this, because the vector terms are both positive and identical. We also know the direction, just by inspection, because of the symmetry of the problem.