Calculate the equation for a line that passes through the points (-2, -1), (4, 8) and (50, p).

What is the value of p? Ok for the points -2,-1 and 4,8 i gotten y=3/2x+2 i don't know how to get the 50.p part please help me

Ok, I agree with your equation of

y = (3/2)x + 2

well, if (50,p) lies on it, shouldn't it satisfy your equation?
then
p = (3/2)(50) + 2
= 75 + 2 = 77

check:
http://www.wolframalpha.com/input/?i=plot+%7B(-2,+-1),+(4,+8)+,+(50,+75)%7D