(6a-4 5b2 c3)-6

Next time, use the power symbol ^ :

[6a^(-4) 5b^2 c^3]^(-6)

Rule 1: (a^x)^y = a^(xy)

Rule 2: a^(-x) = 1 / a^x

Start by multiplying the 6 and 5 inside square brackets:

[30 a^(-4) b^2 c^3]^(-6)

Rule 1 (don't forget that 30 = 30^1):

30^(-6) a^24 b^(-12) c^(-18)

Rule 2:

a^ 24 / (30^6 b^12 c^18)

Kapish?