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