# Help with solving using the sustitution method

• May 20th 2011, 07:42 PM
jay1
Help with solving using the sustitution method
I really could use assistance working through this problem, as my textbook is online and I am having trouble finding the ordered pair using the substitution method.
1) 7x + 4y= -36
2) -2x + y = 21

I realize this is probably simple, but I need help with the steps to solving please. I appreciate it!
• May 20th 2011, 07:50 PM
johnny
1. 7x + 4y = -36, i.e. 4y = -7x - 36.
2. -8x + 4y = 84, i.e. 4y = 8x + 84.
-7x - 36 = 8x + 84, i.e. -36 - 84 = 8x + 7x.
-36 - 84 = -120 and 8x + 7x = 15x, hence 15x = -120.
x = -120/15 = 8
y = 2(-8) + 21 = -16 + 21 = 5
∴ (x, y) = (8, 5)
• May 20th 2011, 07:50 PM
johnny
Edited.
• May 20th 2011, 08:04 PM
jay1
Quote:

Originally Posted by johnny
1. 7x + 4y = -36, i.e. 4y = -7x - 36.
2. -8x + 4y = 84, i.e. 4y = 8x + 84.
-7x - 36 = 8x + 84, i.e. -36 - 84 = 8x + 7x.
-36 - 84 = -120 and 8x + 7x = 15x, hence 15x = -120.
x = -120/15 = 8
y = 2(-8) + 21 = -16 + 21 = 5
∴ (x, y) = (8, 5)

Thank you! You're steps are helping me understand how I reach the answer! You're answer helps me see that I got it righy! I appreciate it