I need to know how to work my way to find this answer, although I know the answer.

Question

The tension T in a spring varies directly as the distance x is stretched. T = 8 lb when x = 2 inches. Find the constant of variation, write the full mathematical model, find the tension when x = 4 inches. 

Answer 1

T = kx

When T = 8 and x = 2:

8 = k2

constant of variation k = 4

full math model; T = 4x

when x = 4:

T = 4(4) = 16