We know sum of interior angles in any triangle is 180°. Your specific triangle is a right triangle (one angle must be 90°) and we are also given another angle (57°) that means we can find angle T,
[tex]T=180^{\circ}-90^{\circ}-57^{\circ}=33^{\circ}[/tex]
Use the sine function to find [tex]RT[/tex],
[tex]\sin(57^{\circ})=15/RT\implies RT=15/\sin(57^{\circ})\approx17.9[/tex]
Use the tangent function to find [tex]RS[/tex],
[tex]\tan(57^{\circ})=15/RS\implies RS=15/\tan(57^{\circ})\approx9.7[/tex]
Hope this helps.
