Question

Let f(x) be a linear function with f(1) = 2 and f(3) = 8. Find f(5) .
Is the final answer 14 or 16

Answer 1

(1,2) and (3,8)
slope = 6/2 = 3
y = 3 x + b
2 = 3 + b
b = -1
so
y = 3 x - 1
if x = 5
y = 15 - 1
which is STILL 14 like Steve told you

Answer 2

If all else fails, make a table

x y
1 2
3 8
5 ?

Each time x changes by 2, y must change by the same amount for the graph to be a straight line.

So, since 8-2=6, 8+6=14