Let additional force required be F.
Vector sum of the four forces and additional force.
F+5i^−6i^+7j^−8j^=0
Force required to make net acceleration zero
F=i^+j^
Therefore, the required force is ∣F∣=12+12=2N
Angle with x-axis: tanθ=xcomponentycomponent=11
Thus, θ=45∘