If the longest side is 6.2cm and is twice as long as the shortest side, then divide the length by 2 to find the shortest side length:
6.2 / 2 = 3.1
Add the longest and shortest: 6.2 + 3.1 = 9.3.
You would then need to add 9.3 to b to get the total perimeter:
9.3+b = 14.5