As per questions,
1)
(3m+7)(9m²- 21m+49)
Expanding to a³±b³ formula,
→ 3m³ + 7³
→ 3³×m³ + 7³
→ 27m³ + 343
2)
(x²-2x)(x⁴-2x³+4x²)
→ x²(x⁴-2x³+4x²)-2x(x⁴-2x³+4x²)
→ x²x⁴-2x²x³+4x²x²-2x×x⁴+2x×2x³-8x×x²
→ x⁶-2x⁵+4x⁴-2x⁵+4x⁴-8x³
3)
(2y²-7y)(4y⁴+ 14y³+49y²)
→ 2y²×4y⁴ + 2y²×14y³ + 2y²×49y² -7y×14y³ -7y×49y²
→ 8y⁶+28y⁵+98y⁴-28y⁵-98y⁴
As
→ 8y⁶-343y³