a)

$7,2:2,88=720:288=\frac{720}{288}=\frac{360}{144}=\frac{180}{72}=\frac{90}{36}=\frac{45}{18}=\frac{5}{2}=2\frac{1}{2}$

$7,2-2,88=7\frac{20}{100}-2\frac{88}{100}=6\frac{120}{100}-2\frac{88}{100}=4\frac{32}{100}=4,32$