quietrain

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
r₁=(-3,0,0)
r₂³ = 4³=64
r₂=(0,0,4)

resultant direction = r₁+r₂=(-3,0,4)
magnitude = (666, 0 , 281)
E-field at origin = (-3*666, 0 , 4*281) N/C