Question 252530
Be careful setting this up. The best way to go here is polynomial division:
{{{5X^3 + 0x^2 + Mx + N}}} / {{{X^2 + X + 1}}}
Dividing carefully gets a quotient of 5X - 5 with a left over part of Mx + N + 5, but the remainder = 0, so,
{{{Mx + N + 5 = 0}}}.
Now set Mx = 0 and solve for M, M = 0
Set (N + 5) = 0 and N = -5.

M + N = 0 + -5 = -5, [C]