f(x)=y
so
f(3)=6 and f(8)=10
the points are
(x,y)
(3,6)
(8,10)
y=mx+b
m=slope
b=yintercept
to find the slope between 2 points, (x1,y1) and (x2,y2)
the slope is (y2-y1)/(x2-x1)
(3,6)
(8,10)
slope=(10-6)/(8-3)=4/5
y=4/5x+b
find b
the equation for a line with slope m and a pointn on it is (x1,y1) is
y-y1=m(x-x1)
a point is (3,6)
y-6=4/5(x-3)
y-6=4/5x-12/5
add 6 or 30/5
y=4/5x+18/5