How can I graph y=-2x+3 and y=-x+3 i the cartesian plane?

Should be easy. These equations describe straight lines.

So, for each equation, determine two points and draw a line between them.

The easiest two points to determine are the x- and y-intercepts.

for y = -2x+3,
x=0 means y = -3
y=0 means x = 3/2

so, plot (0,-3) and (3/2,0)
draw a line between the two points.

Same for the 2nd one.