Find the value of a so that the points A(6, 2a) and B(-a, -2) lie on a line having slope 7.
(2a - - 2) /( 6 - - a) = 7
(2a + 2) / ( 6 + a) = 7
2a + 2 = 7 (6 + a)
2a + 2 = 42 + 7a
2a - 7a = 42 - 2
-5a = 40
a = 40 / -5 = -8
