5) (5a + 3b - 4) 5a =

5) $$(5a + 3b - 4) \cdot 5a = 5a \cdot 5a + 3b \cdot 5a - 4 \cdot 5a = 25a^2 + 15ab - 20a$$

Ответ: $$25a^2 + 15ab - 20a$$