$\text{a)}{x}^2=8^2+{15}^2$  

$x^2=64+225$  

$x^2=289$  

$x=\sqrt{289}=17$  

 

$\text{b)}{y}^2+5^2={12}^2$  

$y^2={12}^2-5^2$  

$y^2=144-25$  

$y^2=119$  

$y=\sqrt{119}$  

 

$\text{c)}{z}^2=7^2+5^2$  

$z^2=49+25$  

$z^2=74$  

$z=\sqrt{74}$