__Example 2__Find the value of the following.

(a) ${\mathrm{log}}_{2}7+{\mathrm{log}}_{2}12-{\mathrm{log}}_{2}21$

(b) $3{\mathrm{log}}_{10}5+2{\mathrm{log}}_{10}2-{\mathrm{log}}_{10}5$

(c) $2{\mathrm{log}}_{10}3-{\mathrm{log}}_{10}3+{\mathrm{log}}_{10}3\frac{1}{3}$

(d) ${\mathrm{log}}_{3}3p+{\mathrm{log}}_{3}3q-{\mathrm{log}}_{3}pq$