If $\displaystyle {\tan{\alpha}}=-\frac{{3}}{{8}}$ such that $\displaystyle \alpha$ terminates in quadrant II and $\displaystyle {\sin{\beta}}=\frac{{4}}{{5}}$ such that $\displaystyle \beta$ terminates in quadrant II then

 $\displaystyle {\cos{{\left(\alpha-\beta\right)}}}=$Preview Question 6  

Your answer should be exact but you do not have to rationalize your denominator.