Answer:
15, 6
Explanation:
x = large number
y = small number
1) x + y = 21
2) x = 21 - y
3) 2x - 2y = 18
substitute equation 2 into equation 3 so you have an expression all in terms of y, then you can solve for y.
2(21-y) - 2y = 18
42 - 2y - 2y = 18
-4y = -24
y = -24/-4 = 6
Now you can solve for x, plug y=6 into equation 1 :
x + 6 = 21
x = 21 - 6 = 15