A straight DNA is 9000 base pairs long. Enzyme A cuts it at 1500 bp and at 4200 bp, measured from…

Step 1. (a) Enzyme A: $1500$, $4200 - 1500 = 2700$, and $9000 - 4200 = 4800$. That is $3$ fragments, and the largest is $4800$ bp.

Step 2. (b) Enzyme B: $2600$, $6100 - 2600 = 3500$ and $9000 - 6100 = 2900$. That is $3$ fragments, and the largest is $3500$ bp.

Step 3. (c) The cuts in order: $1500$, $2600$, $4200$, $6100$. Fragments: $1500$, $1100$, $1600$, $1900$ and $2900$ bp. That is $5$ fragments. The smallest is $1100$ bp.