You have the answer if you just do a little bit more but other than that you have your answer
Number of strings =

= 21 x 21 x 21 x 21 x 21 x 5 = 20 420 505
Answer: 20 420 505
Multply both equations by 2 leaving
x + 2y = 8
x - 2y = -2
Add the equations giving
2x = 6
x = 3
Substitute 3 in the top equation
3 + 2y = 8
2y = -5
y = -2.5
-3(x+5)=-9
-3x-15=-9
+15 +15
-3x=6
-1x=2
1x=-2
x=-2