To find hypotenuse $ c $ use formula:
$$ \sin \left( \alpha \right) = \dfrac{ a }{ c } $$After substituting $ \alpha = 45^o $ and $ a = 7 $ we have:
$$ \sin( 45^o ) = \dfrac{ 7 }{ c } $$ $$ \frac{\sqrt{ 2 }}{ 2 } = \dfrac{ 7 }{ c } $$ $$ c = \dfrac{ 7 }{ \frac{\sqrt{ 2 }}{ 2 } } $$ $$ c = 7 \sqrt{ 2 } $$