The variables x=-6 and y=15 varies directly. How do you write an equation that relates the variables and find y when x=8?
Solve a proportion.
If x and y vary directly, it means y = kx, where k is the constant of proportionality.
For any pair (x, y), we have
We have (-6, 15) as one set of values, and we have (8, y) as another -- where y is unknown. Solve the proportion:
Finish the problem by "cross-multiplying", and solving for y. You should get y = -20.