`a)` `( 3/8 − 3/4 + 7/12 ) : 5/6 + 1/2`

`= ( 9/24 − 18/24 + 14/24 ) : 5/6 + 1/2`

`= 5/24 : 5/6 + 1/2`

`= 5/24 × 6/5 + 1/2`

`= 6/24 + 1/2`

`= 1/4 + 1/2`

`=>` ` 3/4 `

`------`

`b)` `( 8/51 + 12/34 − 11/17 ) : ( 11/30 − 12/45 )`

`= ( 8/51 + 6/17 − 11/17 ) : ( 11/30 − 4/15 )`

`= ( 8/51 − 5/17 ) : ( 11/30 − 8/30 )`

`= ( 8/51 − 15/51 ) : 3/30`

`= −7/51 : 1/10`

`= −7/51 × 10`

`=> −70/51`

$\color{#FF4500}{\diamond\triangle\star}~\color{#FF0000}{ng}\color{#FF6600}{uy}\color{#FFCC00}{en}\color{#33CC33}{p}\color{#33CCFF}{h}\color{#6633FF}{uc}\color{#CC33FF}{th}\color{#FF3399}{an}\color{#FF3366}{g}\color{#FF0033}{26}\color{#FF6600}{04}~\color{#FF4500}{\star\triangle\diamond}$