T todd5821 New member Joined Jul 4, 2007 Messages 3 Jul 4, 2007 #1 Here is the problem: 5a + 2b = -16 a + 3b = 15 I understand to multiply the second by 5, but when both equations have the same sign (add or subtract) I am having difficulty.

Here is the problem: 5a + 2b = -16 a + 3b = 15 I understand to multiply the second by 5, but when both equations have the same sign (add or subtract) I am having difficulty.

D Denis Senior Member Joined Feb 17, 2004 Messages 1,470 Jul 4, 2007 #2 5a + 2b = -16 a + 3b = 15 Multiply by -5 instead: 5a + 2b = -16 -5a - 15b = -75 Now add: -13b = -91 That's a suggestion; I prefer adding to subtracting.

5a + 2b = -16 a + 3b = 15 Multiply by -5 instead: 5a + 2b = -16 -5a - 15b = -75 Now add: -13b = -91 That's a suggestion; I prefer adding to subtracting.

T todd5821 New member Joined Jul 4, 2007 Messages 3 Jul 4, 2007 #3 I have done that, but to solve for b I would divide by 13. 75 divided by 13 doesn't come out as a whole number. The answer book states that a=-6 and b=7. I haven't figured out how they got that!!

I have done that, but to solve for b I would divide by 13. 75 divided by 13 doesn't come out as a whole number. The answer book states that a=-6 and b=7. I haven't figured out how they got that!!

M Mrspi Senior Member Joined Dec 17, 2005 Messages 2,128 Jul 4, 2007 #4 Please re-read Denis's earlier post. When you add the two equations together, you don't get -75.....you get -13b = -91 Now....divide both sides by -13.

Please re-read Denis's earlier post. When you add the two equations together, you don't get -75.....you get -13b = -91 Now....divide both sides by -13.