Consider the line passing through two points P=(1,8) and Q=(−1,3). If the line also passes through point R=(15,a), what is the value of a?
slope PQ = (3-8)/(-1-1) = 5/2
slope PR = (a-8)/(15-1)
(a-8)/14 = 5/2
2a - 16 = 70
2a=86
a = 43
or
you could find the equation of the line passing through P and Q, then sub in the point R to find a
(you should get equation
5x-2y = -11
plugging in (15,a) yields the value of a = 43