Question

which vector is orthogonal to u=)(-9,3)?

Answer 1

the condition of orthogonality is the dot product = 0
u=(9,3) v=(a,b) (9 x a) + (3 x b) =0

therefore there are many results

a=1 b=-3
a=-1 b=3
a=2 b=-6
a=10 b=-30
etc...