Replace x with -3 and add
y = -x+1
y = -( x ) + 1
y = -( -3 ) + 1 .... replace x with -3
y = 3 + 1 ... two negatives cancel to make a positive
y = 4
So the number 4 goes right next to -3. This means the point (x,y) = (-3,4) is on the line. This is shown as point A on the attached image below.
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Repeat for x = -1
y = -x + 1
y = -( x ) + 1
y = -( -1) + 1 ... replace x with -1
y = 1 + 1
y = 2
So the point (-1,2) is also on the line. This is shown as point B.
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Repeat for x = 0
y = -x + 1
y = -0 + 1 ... replace x with 0
y = 1
So (0,1) is on the line. This is point C.
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Finally, plug in x = 4
y = -x + 1
y = -4 + 1 ... replace x with 4
y = -3
So (4,-3) is on the line, shown as point D.
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After you have at least 2 points plotted on the coordinate grid, you would draw a straight line through those points. Note how points A,B, C, & D are all on the same straight line. The slope of this line is -1 meaning you go down 1 unit each time you move to the right 1 unit. The y intercept is 1, which is where the blue line crosses the y axis.