I need help, I don't understand this, so please try to explain?

Yeah, so basically the image is showing restriction enzymes. The job of restriction enzymes is mainly involved in research when scientists use them for cloning human genes. But that's besides the point...

Main thing you have to understand is that restriction enzymes cut at very specific places along DNA sequences. If you look at the restriction enzyme Rsa 1, you can notice that it cuts only between a thymine nucleotide base and an adenine nucleotide base. Next, if ya look at Sty 1 (be careful b/c W can represent adenine or thymine), it cuts only between two directly adjacent cytosine nucleotide bases!

SO if we go to Rsa 1, we can find the answers by dividing up the sequences between the pattern we saw in the gray box. It only cuts between adenine and thymine bases. Based on that, we can find the number of fragments created, and the segment lengths (basically just like how many nucleotide bases are in each strand). Hope ya found this helpful!

a and b

i think a) betsy can donate only blood to mark is correct because, mark and betsy have the same blood type. whereas nobody else has b+ blood so betsy can't donate to them.