This is the first time I've ever posted here, but I think people may be trying too hard for 10ai.
y = mx + b
The gradient of this line is m. The points (a,a(squared)) and (B,B(squared)) lie on the line, so:
m = {B(squared) - a (squared)} / {B - a}
= {(B-a)(B+a)/(B-a)
= B + a...