Given:

y varies directly with x i.e. y is proportional to x.

⇒ y = kx, where k is some real number. ------------- (i)

Given y = 12 at x = 3

Putting x = 3 and y = 12 in eq. (i)

We have,

12 = 3k

⇒ k = = 4

Thus the given linear equation is y = 4x

At x = 5: y = 4 x 5 = 20.