Respuesta :
x = tee shirts, y = long sleeved shirts
x + y = 9....x = 9 - y
10x + 7y = 72
10(9 - y) + 7y = 72
90 - 10y + 7y = 72
-10y + 7y = 72 - 90
-3y = - 18
y = -18/-3
y = 6 <=== 6 long sleeved shirts
x + y = 9
x + 6 = 9
x = 9 - 6
x = 3 <== 3 tee shirts
x + y = 9....x = 9 - y
10x + 7y = 72
10(9 - y) + 7y = 72
90 - 10y + 7y = 72
-10y + 7y = 72 - 90
-3y = - 18
y = -18/-3
y = 6 <=== 6 long sleeved shirts
x + y = 9
x + 6 = 9
x = 9 - 6
x = 3 <== 3 tee shirts