Let f be a linear function such that f(2) =5 and f(6) = -1. find an equation for f(x).

Using f(x) = y, we know that a graph of the function contains the (x,y) points (2,5) and (6,-1). first find the slope of that line,
m = (y2 - y1)/(x2 - x1) ⇒ -6/4⇒-3/2

then using either point (I'll use the first one) solve for b in y = mx + b.
5 = (-3/2)(2) + b⇒ 5 = -3 + b⇒ 8 = b.

So y = (-3/2)x + 8 ⇒ f(x) = (-3/2)x + 8.