Maggie's home computer downloads a
7 megabyte program in 5/6 minute.
Brooke's home computer takes 1 1/4 minutes
to download a 9 megabyte program.
Whose computer downloads at a greater
rate of megabytes per minute?

In this question, Maggie's home computer has greater download rate of megabytes in each minute.

Explanation:

Maggie's home computer downloads 7-megabyte program in 5/6 minute

=> In each minute, it can download a program with: 7 / 5 x 6 =8.4 megabytes

Similarity, Brooke's home computer downloads 9-megabyte program in 1 1/4 minute (1 1/4 minute = 5/4 minute)

=> In each minute, it can download a program with: 9/5 x 4 = 7.2 megabytes

As 8.4 > 7.2

So that Maggie's home computer has greater download rate of megabytes in each minute.

No  problem !  You came to the right place.

You said that                                    3y - 15x = 12

Add  15x  to each side of the equation:    3y = 15x + 12

Divide each side by  3 :                              y =  5x  +  4

Probably easier than you expected.