90. Even or odd product

$$
(7 a+b-2 c+1)(3 a-5 b+4 c+10)
$$

where the numbers $a, b, c$ are integers
$\triangle$ We can consider cases related to the parity of numbers $a, b$, and $c$ (8 cases!), but it is simpler to proceed differently. Let's add the factors:

$$
(7 a+b-2 c+1)+(3 a-5 b+4 c+10)=10 a-4 b+2 c+11
$$

Since the resulting sum is odd, one of the factors of the given product is even, and the other is odd. Therefore, the product itself is even.

Answer: even.