Use ^ and then the number to designate powers.
-x^2 + 8x - 12
-1(x^2 - 8x + 12)
The x-2 in the denominator can canceled with the x-2 in the numerator and you are left with -1(x-6).
(2a^3 + a^2 - 3a)/(6a^3 + 5a^2 - 6a)
Factor out a out of the numerator and denominator and you are left with
(2a^2 + a - 3)/(6a^2 + 5a - 6)
2x+3 in the numerator and denominator cancels out and you are left with (x-1)/(3x-2).
All of those problems can be done with factoring of the numerator and denominator and cancel out the common factors.