the length would be 10 inches.
to find perimeter, you add length+length+width+width.
since both widths and both lengths have to be the same, u add the width 2 times or 7+7=14.
next u subtract 14 from 34 or 34-14=20.
since u still have 2 lengths that are equal, u divide 20 by 2 or 20/2=10
hope this helps! :)
