\({v^2} = 400\)
\(\Rightarrow \,v = 20\,m/s\).
\(acceleration \,(a)\,=\, -\, g \,sin\alpha\, =\,-\, g\, sin 45^o,\)
\({(20)^2} = {u^2} - 2\frac{g}{{\sqrt 2 }}.20\sqrt 2\) \([v^2 = u^2 + 2as]\)
\({u^2} = {20^2} + 400\)
\(\Rightarrow \,u\,= \,20\sqrt 2 \,m/s\).