Roger started with 90 items in his MP3 collection. Each month, he buys 15 more MP3s to add to his collection. The number of MP3s
in his collection, P, is given by P=15m + 90, where m is the number of months he has added 15 MP3s to his collection
How many MP3s will be in Roger's account in 9 months?

x = -3

y = -18

step-by-step explanation:

to solve by substitution, substitute one variable and solve for the other. then substitute again to find the remaining variable.

x-1/3(3x-9) = -3 since y=3x-9 substittute y.

x-1/3(3x-9)=-3

x + x +3 = -3

2x+3=-3

2x+3-3= -3-3

2x=-6

x=-3

substitute again into y=3x-9

y=3(-3)-9

y=-9-9

y=-18