Since the coefficients are rational numbers, if one answer is a complex number, the other answer will be its conjugate.
The conjugate of 4 + 3i is 4 - 3i.
Therefore, the problem, when factored will become: [ x - (4 + 3i) ][ x - (4 - 3i) ] = 0
Multiplied out, it becomes: x2 - 8x + 25 = 0, which means that k = 8.