we have that
x + y = 10
step 1
find the x and y intercepts coordinates
x intercept
for y=0
x + y = 10------> x=10
the point is (10,0)
y intercept
for x=0
x + y = 10------> y=10
the point is (0,10)
step 2
graph the equation with the two points obtained
using a graph tool
see the attached figure
