ii) the electric field at a point is the superposition of the point charges everywhere else
so question wants at origin.
at origin, the E-field from +Q1 is towards the -ve x direction.
at origin, the E-field from -Q2 is towards the +ve z direction.
so your resultant direction is not (6,0,4.5), i think your magnitude is wrong too
E = kQ1 r1 / r13 + -kQ2r2 /r23
so k = 9x109
Q1 = +2x10-6
Q2 = -2x10-6
r13 = 33=27
r23 = 43=64
resultant direction = r1+r2=(-3,0,4)
magnitude = (666, 0 , 281)
E-field at origin = (-3*666, 0 , 4*281) N/C