I'll start you off (tell me if I've got some kind of misconception),Originally Posted byaskmemath

you know two points on the graph: (0,10) and (80,28)

use that information to find the equation.

once you have the equation, substitute 20 for y (or f(x), whichever you prefer) and solve for x

to find the load that makes the spring reach it's limit, substitute 40 for y and solve for x