Answer:
(b) as marked
Step-by-step explanation:
Subtract r, multiply by u^2/(-k-y-r), and take the square root.
[tex]\dfrac{m}{u^2}+r=-k-y\qquad\text{given}\\\\\dfrac{m}{u^2}=-k-y-r\qquad\text{subtract $r$}\\\\\dfrac{m}{-k-y-r}=u^2\qquad\text{multiply by $\dfrac{u^2}{-k-y-r}$}\\\\u=\pm\sqrt{\dfrac{m}{-k-y-r}}\qquad\text{take the square root}[/tex]
