1. **Problem (a): Factorise completely** the expression $$8xy - 28y - 6x + 21$$. 2. Group terms to factor by grouping: $$ (8xy - 28y) - (6x - 21) $$ 3. Factor out common factors from each group: $$ 4y(2x - 7) - 3(2x - 7) $$ 4. Notice the common binomial factor $ (2x - 7) $, factor it out: $$ (2x - 7)(4y - 3) $$ --- 5. **Problem (b): Simplify** the expression $$\frac{3}{a - 4} - \frac{2a}{(a - 4)^2}$$. 6. Find a common denominator, which is $$ (a - 4)^2 $$. 7. Rewrite the first fraction with the common denominator: $$ \frac{3}{a - 4} = \frac{3(a - 4)}{(a - 4)^2} $$ 8. Substitute back: $$ \frac{3(a - 4)}{(a - 4)^2} - \frac{2a}{(a - 4)^2} = \frac{3(a - 4) - 2a}{(a - 4)^2} $$ 9. Simplify the numerator: $$ 3(a - 4) - 2a = 3a - 12 - 2a = a - 12 $$ 10. Final simplified expression: $$ \frac{a - 12}{(a - 4)^2} $$