soccer ball = a = 5
basketball = b = 9
volleyball = c = 7
a + b + c = 21
2a + c = 17
2b + c = 25
2a + b + c = 26
Seeing that....
a + b + c = 21
2a + b + c = 26
We can deduce that a = 5
2a + c = 17
2(5) + c = 17
10 + c = 17
Subtract 10 from both sides
c = 7
2a + b + c = 26
2(5) + b + 7 = 26
10 + b + 7 = 26
17 + b = 26
Subtract 17 from both sides
b = 9
Hope this helped!