Solve for a
a\in \left(-\infty,-1\right)\cup \left(2,\infty\right)
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-a-1>0 2a-4<0
For the product to be negative, -a-1 and 2a-4 have to be of the opposite signs. Consider the case when -a-1 is positive and 2a-4 is negative.
a<-1
The solution satisfying both inequalities is a<-1.
2a-4>0 -a-1<0
Consider the case when 2a-4 is positive and -a-1 is negative.
a>2
The solution satisfying both inequalities is a>2.
a<-1\text{; }a>2
The final solution is the union of the obtained solutions.
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