(the vectors in the problem are really in column form, but i don't know how to type that so i put them in component form)

vectorP = 2i +5j

vectorQ = -i+ 2j

☆find the value of p·q

(I know the dot product is 8, that was ez) but the next part i don't get...

☆ if... s = kp - (p·q)q

find the value of the constant k such that s is perpendicular to q

plz help