Question 1199801
(a^2-3a-4)/(a^2-4) x (2-a-a^2) /(a^2-1)


{{{((a^2-3a-4)/(a^2-4))((2-a-a^2) /(a^2-1))}}}

You need to know how to factorize degree-two trinomials and difference of squares.  (and understand convenience of -1 as a factor).


{{{((a^2-3a-4)(-1)(a^2+a-2))/((a^2-4)(a^2-1))}}}


{{{((a-4)(a+1)(-1)(a+2)(a-1))/((a+2)(a-2)(a+1)(a-1))}}}


Look for and cancel factors of 1, numerator and denominator same factor

{{{((a-4)cross((a+1))(-1)cross((a+2))(a-1))/((a+2)cross((a-2))cross((a+1))(a-1))}}}


{{{((-1)(a-4)(a-1))/((a+2)(a-1))}}}


{{{((-1)(a-4)cross((a-1)))/((a+2)cross((a-1)))}}}


and then,.....!

PROBLEM:  There appears to be one or two mistakes there.  Pure text is very difficult to work with.  This kind of arithmetic works better on paper with pencil.