X:y= 5:3 and x+y =56 workout the value of x-y

I only got 2/3 marks for:x + y = 56 (1)
x:y = 5:3
x/y = 5/3
3 x = 5 y
so, x = (5/3)y(2)
sub (2) in (1)
x + y = 56
(5/3)y + y = 56
5 y + 3 y = 168
8 y = 168
y = 168 / 8 = 21
x = (5/3) x 21
= 35
the value of x - y = 35 - 21
= 14
Please help