a) $$(5-2\frac{3}{7}):\frac{21}{36}=(\frac{5*7-2*7-3}{7}):\frac{21}{36}=(\frac{35-14-3}{7}):\frac{21}{36}=\frac{18}{7}:\frac{21}{36}=\frac{18*36}{7*21}=\frac{6*12}{7*7}=\frac{72}{49}$$