Glencoe Algebra 2, Chapter 2, 2.3, Section 2.3, Problem 48

Given the line passes through (0,0) , parallel to graph of x+y=10
as the slope of the line x+y=10 is as follows
x+y=10
=> y = 10 -x
=> y = -x +10
this is of the form y= mx +c where m is the slope
so the slope of the line x+y=10 is m = -1
so the line passing through the point (x,y)=(0,0) is also -1 as both the lines are parallel
then the line equation passing through (0,0) is follows
y= mx+c
0 = (-1)(0)+c
0= 0 +c
c = 0
so the desired equation of the line is
y = -x +0
=> y = -x or x+y =0
and it is plotted and in the attachments and the point (0,0) is plotted with a blue dot