Using the information on part a Interpret the meaning If the quotient in terms of the two fractions given

Expresa commuter train leaves downtown sation and travels at a average speed of 55 miles per hour towards the north side sation, which is 50miles away. thirty minutes later, express commuter train #7 leaves north side and travels at a average speed of 35miles per hour towards downtown sation. at the moment the two trains pass each other, how far(in miles) is train #12 from the downtown sation and how long(in minutes) has the #12 train been traveling

Can someone me out here number 4 plz

Bring the fraction a/a-4 to a denominator of 16-a^2