let y = ax+b
(2,-4)
-4 = 2a+b
b= -4-2a ----(1)
(4,1)
1=4a+b
= 4a-4-2a ( from (1))
2a = 5
a = 5/2
b= -4-2(5/2) = -9
一次函数:y = (5/2)x - 9