So this question is basically asking for the highest common factor of 72 and 90.
You can work this out by looking at the prime factors of these numbers.
For 72, the prime factors are

or

For 90, the prime factors ar

or

We are interested in the prime factors which are repeated. In this case, that's

or


so the highest common factor, and so the longest strips which could be cut from both 72 inches and 90 inches, is 18 inches.
I hope that helped.