First, set up the two different equations.
f=first number
s=second number
2f - s = 18
2s - f = 9
Second, solve the equations by multiplication which will cancel out the s's.
2(2f - s = 18)
1 (2s - f = 9)
___________
4f -2s = 36
2s - f = 9
___________
4f - f = 45
3f = 45
f = 15
So the first number is 15.
Plug 15 into one of the equations to find the second number.
2f - s = 18
2(15) - s = 18
30 - s = 18
-s = -12
s = 12
Overall, the first number is 15 and the second number is 12.
Check:
2f - s = 18
2(15) - 12 = 18
30 -12 = 18
18 = 18 True
2s - f = 9
2(12) - 15 = 9
24 - 15 = 9
9 = 9 True
