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Answer:
u = ±√(-m/(k+r+y))
Step-by-step explanation:
Subtract r, rearrange, then take the square root.
[tex]\dfrac{m}{u^2}+r=-k-y \qquad\text{given}\\\\ \dfrac{m}{u^2}=-k-y-r\\\\-\dfrac{m}{k+r+y}=u^2\\\\ \boxed{u=\pm\sqrt{-\dfrac{m}{k+r+y}}}[/tex]
