A vector that is orthogonal to #2# other vectors is calculated with the cross product. The latter is calculate with the determinant.

#| (veci,vecj,veck), (d,e,f), (g,h,i) | #

where #veca=〈d,e,f〉# and #vecb=〈g,h,i〉# are the 2 vectors

Here, we have #veca=〈-4,-5,2〉# and #vecb=〈4,4,2〉#

Therefore,

#| (veci,vecj,veck), (-4,-5,2), (4,4,2) | #

#=veci| (-5,2), (4,2) | -vecj| (-4,2), (4,2) | +veck| (-4,-5), (4,4) | #

#=veci((-5)*(2)-(4)*(2))-vecj((-4)*(2)-(4)*(2))+veck((-4)*(4)-(-5)*(4))#

#=〈-18,16,4〉=vecc#

Verification by doing 2 dot products

#〈-18,16,4〉.〈-4,-5,2〉=(-18)*(-4)+(16)*(-5)+(4)*(2)=0#

#〈-18,16,4〉.〈4,4,2〉=(-18)*(4)+(16)*(4)+(4)*(2)=0#

So,

#vecc# is perpendicular to #veca# and #vecb#

The unit vector is

#hatc=(vecc)/(||vecc||)#

The magnitude of #vecc# is

#||vecc||=||〈-18,16,4〉||=sqrt((-18)^2+(16)^2+(4)^2)#

#=sqrt(596)#

The unit vector is #1/sqrt(596)*〈-18,16,4〉#