Expanding the given expressions:

2a(3a-5)-(a-3)(a-7)
= 6a^2 - 10a - (a^2 - 10a + 21)
= 6a^2 - 10a - a^2 + 10a - 21
= 5a^2 - 21

(c+3)(5-c)-3c(1-c)
= 5c - c^2 + 15 - 3c + 3c^2
= -c^2 + 3c + 15