Formula of the point-slope equation for the right line that passes through two points is
y-y1=(y2-y1)/(x2-x1)*(x-x1) In our case (x1,y1)=(-10,-20) and (x2,y2)=(1,-9)
y-(-20)=(-9-(-20))/(1-(-10))*(x-(-10)) => y+20=(-9+20)/(1+10)*(x+10) =>
y+20= 11/11*(x+10) => y+20=x+10 => y=x-10
Good luck!!!