Math

-1(-2)(-3)(-3)

Since -1·-2  =  2:                        (a negative times a negative is a positive)

=  2(-3)(-3)

Since 2·-3  =  -6:                        (a positive times a negative is a negative)

=  -6(-3)

Since -6·-3  =  18

=  18

$(-1)(-2)(-3)(-3)\\ = -1(-1)(-1)(-1)(2)(3)(3)\\ = 2(3)^2\\ = 18$

Same as Geno's answer.