$a)$  

$\sin {\left(-75\right)}^{\circ }=-\sin {75}^{\circ }=-\frac{\sqrt{6}+\sqrt{2}}{4}$ $\sin {\left(-75\right)}^{\circ }=-\sin {75}^{\circ }=-\frac{\sqrt{6}+\sqrt{2}}{4}$    

 

$b)$  

$\sin {15}^{\circ }=-\left(-\sin {15}^{\circ }\right)=-\left(\sin {\left(-15\right)}^{\circ }\right)=-\frac{\sqrt{2}-\sqrt{6}}{4}$