In non-specific terms, when I say a varies directly with b, all I’m saying is that changing a changes b in the same way. For instance, if I double a, I’ll double b as well. If I halve a, I’ll halve b. There’s a symbol for direct variation which I can’t repeat here, but and direct variation lends itself to an equation. In the case of a varies directly as b, the equation will be

a = kb

where k is the constant of variation.

In your problem, s varies directly as p, so

s = kp

where k is a constant. You’re told that s = 5.6 when p = 80, so

5.6 = 80k

k = 5.6/80 = .07

So, s = .07 p

In practical terms, this means that the sales tax is 7% of the price.

I hope that helps.