Since you have exponents this is what you end up getting.
I only used the calculator for the multiplying. (Also I used * for multiplying)
(12x^4+5x^3+3x^2-5)/(x+1)
(12x*x*x*x+5x*x*x+3x*x-5)/(x+1)
(12+5+3-5)/(x+1)
(15)/(x+1)
You divide.
You get 15.
Now you add up the x's.
15x^9 plus another x because of the one in the other parentheses.
Answer: 15x^10