#### Answer

I-90 = 178.5 + m

#### Work Step by Step

I-80 length is m miles. Also, I-80 length is 178.5 miles shorter than I-90. This means that I-90 length is 178.5 miles longer than I-80. Since I-80 length is m miles, then I-90 is 178.5 miles longer than m.
So. I-90 = 178.5 + m