Question 1042899
{{{A=21+B}}}
{{{C=2B-17}}}
and 
{{{A+B+C=180}}}
{{{(21+B)+B+(2B-17)=180}}}
{{{4B+4=180}}}
{{{4B=176}}}
{{{B=44}}}
Then,
{{{A=21+44=65}}}
{{{C=2(44)-17=71}}}