Divide the perimeter by 2, this will give you the sum of one length and one width.
68/2 = 34 yards
Now the length is 10 yards more, so subtract 10 from the 34 and divide by 2 again:
34 - 10 = 24
24 /2 = 12
The width is 12 yards.
The length is 12 + 10 = 22 yards.