To find angle $ \gamma $ use formula:
$$ \alpha + \beta + \gamma = 180^o $$After substituting $ \alpha = 42^o $ and $ \beta = 36^o $ we have:
$$ 42^o + 36^o + \gamma = 180^o $$ $$ \gamma + 78^o = 180^o $$ $$ \gamma = 180^o - 78^o $$ $$ \gamma = 102^o $$