Answer:
(7 a - 8y^2) x (3-y)
Step-by-step explanation:
21a+8xy^3 −24y^2−7axy
factor out 3 from the expression
3( 7 a - 8 y^2) + 8y^3 - 7 a y
3(7 a - 8 y^2) - y x (-8y^2 + 7 a)
Factor out 7 a - 8y^2 from the expression
(7 a - 8y^2) x (3-y)
= (7 a - 8y^2) x (3-y)