STEP 1: find side $ b $
To find side $ b $ use formula:
$$ A = \dfrac{ a \cdot b }{ 2 } $$After substituting $ A = 90 $ and $ a = 3 $ we have:
$$ 90 = \dfrac{ 3 \cdot b }{ 2 } $$$$ 90 \cdot 2 = 3 \cdot b $$$$ 180 = 3 \cdot b $$$$ b = \dfrac{ 180 }{ 3 } $$$$ b = 60 $$STEP 2: find angle $ \beta $
To find angle $ \beta $ use formula:
$$ \tan \left( \beta \right) = \dfrac{ b }{ a } $$After substituting $ b = 60 $ and $ a = 3 $ we have:
$$ \tan \left( \beta \right) = \dfrac{ 60 }{ 3 } $$ $$ \tan \left( \beta \right) = 20 $$ $$ \beta = \arctan\left( 20 \right) $$ $$ \beta = 87.1376^o $$