elem.num.theo - diophantine equations

define all solutions in integers to diophantine equations:

56x+72y=40

ok - I'll write my steps, my problem is that I end up with no solutions at all - maybe someone can find where I went wrong ... ?

SO:

72 = 56(1) + 16

56 = 16(3) + 8

16 = 8(2)

so gcd(56,72) = 8

next:

8 = 56 - (16)(3)

= 56 - 3(72-56)

= 56(4) + 72(-3)

40 = 56(20) + 72(-15)

so x0=20 & y0=-15

so:

x = 20 + 9t, for some int. t

y = -15 - 7t, for some int. t

and I get:

-(15/7) > t > -(20/9)

but there are no integers that work.

did I do something stupid in algebra or can there really be no solutions?