Answer :

naǫ
[tex]21 \frac{15}{16}-18 \frac{3}{4}=21 \frac{15}{16} - 18 \frac{3 \times 4}{4 \times 4}=21 \frac{15}{16}-18 \frac{12}{16}=\boxed{3 \frac{3}{16}}[/tex]