Question 906744
x+y=180, because given as supplementary


x=-20+19y, translating the description


Substitute for x and solve first for y.
(What I mean is, the two formed equations are enough information and you can substitute for x.  That is the basic process; NOT the finished answer.)


Further commenting:
You are supposed to do this...
{{{x+y=180}}}
{{{(-20+19y)+y=180}}}
{{{-20+19y+y=180}}}
{{{20y-20=180}}}
{{{20y=200}}}
{{{highlight(y=10)}}}
and then you should know what else to do to find the value for x.