To find angle $ \beta $ use formula:
$$ \alpha + \beta = 90^o $$After substituting $ \alpha = 120^o $ we have:
$$ 120^o + \beta = 90^o $$ $$ \beta = 90^o - 120^o $$ $$ \beta = -30^o $$The result has to be greater than zero. $ \Longrightarrow $ The problem has no solution.