smaller leg = x
longer leg = y
hypotenuse = z
y=x+7
z=x+8
due to 
[tex] {z}^{2}  =  {x}^{2}  +  {y}^{2}  \\ (x + 8) ^{2}  =  {x}^{2}  + (x + 7) ^{2}  \\  {x}^{2}  + 16x + 64 =  {x}^{2}  +  {x}^{2}  + 14x + 49 \\  {x}^{2}  + 16x + 64 = 2 {x}^{2}  + 14x + 49 \\  {x}^{2}  - 2 {x}^{2}  + 16x - 14x + 64 - 49 = 0 \\  -  {x}^{2}  + 2x + 15 = 0 \\  {x}^{2}  - 2x - 15 = 0 \:  \: (multiplied \: by \:  - 1) \\ (x - 5)(x + 3) = 0 \\ then \:  \: x = 5 \:  \: or \:  \: x =  - 3[/tex]
but x cannot be -3 
so smaller leg is 5 inches
then longer leg = 5+7 = 12 inches
and hypotenuse = 5+8 = 13 inches