Can anyone assist me in these 2 questions(Hi)

1. Find all values of a so that the distance between points (A,-9) and (-2A,7) is 20 units. I did this

Square root (7-(-9))^2+(-2a-a)^2

I get square root 256+9A=20 But how would I proceed taking the square root and the squaring 20?

My next question I am not sure how to start

Collinear points lie on the same line. Find the value of K for which the points (15,1)

(-3,8) and (3,k) are collinear.