If one side of the rectangle is 8 inches, that means the side opposite of it is also 8 inches.
Add those together and subtract it from 30.
8+8=16, 30-16= 14.
You have 14 inches left for the two remaining sides.
Divide 14 by two, which will give you seven.
14÷2=7.
The length is 7 inches.
I hope that helps!