Let the adjacent sides be 17x and 7x respectively.
Second side = 7x, but second side = 3.5cm (given)
So, 7x cm = 3.5 cm
x = 3.5/ 7
= 35/ 7 * 10
= 5/ 10
= 0.5
So, x = 0.5
So 1st side = 17x = 17 * 0.5 = 8.5 cm
2nd side = 7x = 7 * 0.5 = 3.5 cm ( which is already given)
So, perimeter = 2 ( l + b) =
2 ( 8.5 + 3.5) cm
= 2 (12)
= 24
So, Perimeter = 24 cm