$$ 3 \dfrac{5}{12} - 1 \dfrac{2}{3} $$
It can be written as
$$ = \dfrac{12 \times 3 + 5}{12} - \dfrac{3 \times 1 + 2}{3} $$
$$ = \dfrac{41}{12} - \dfrac{5}{3} $$
LCM of $$12$$ and $$3$$ is $$12$$
$$ = \dfrac{41 \times 1}{12 \times 1} - \dfrac{5 \times 4}{3 \times 4} $$
By further calculation
$$ = \dfrac{41 - 20}{12} $$
$$ = \dfrac{21}{12} $$
$$ = \dfrac{7}{4} $$
$$ = 1 \dfrac{3}{4} $$