Vector Operations Dot Product

Let a=

∧

i

+2

∧

j

−

∧

k

,b=

∧

i

−

∧

j

and c=

∧

i

−

∧

j

−

∧

k

be three given vectors. If r is a vector such that r×a=c×a and r⋅b=0, then r⋅a is equal to ......... .