For
f(x) = 5(x+4)/x2 + x - 12
Factor
The numerator needs no further factoring
The denominator does need factoring
Denominator Factors as follows:
(x + 4)(x - 3)
f(x) simplifies to
5(x + 4)/(x + 4)(x - 3)
Canceling
the factor (x + 4) is in the numerator and denominator so it can be canceled, crossed out.
f(x) simplifies further to
f(x) = 5/(x -3)
You can't have division by 0, therefore, x cannot equal positive 3
f(x) = 5/-3 when x is zero
Remember you can graph your function at Desmos.com as y = 5/(x - 3). Note positive 3 is a barrier or asymptote. The x axis is also a barrier.
I hope this helps.