Malaygoel, it's not that your wrong it's just that the answer needs to be in simplest radical form, otherwise it would've taken maybe 4 seconds to do.7.96(2.4494 - 2)

Also,

where did you get $\displaystyle \sqrt{3.99} $?$\displaystyle \frac{15.92}{2} (\sqrt{ 6} - \sqrt{ 4})=

\sqrt{3.99}\sqrt{ 6} - \sqrt{3.99}\sqrt{ 4}$

Remember: $\displaystyle \frac{15.92}{2}=7.96=\sqrt{63.3616} $