The variable x is a placeholder for a number
Let's say we replace x with 0
y = 2x+3
y = 2(0)+3
y = 0+3
y = 3
This means x = 0 and y = 3 pair up together. In short, the point (0,3) is on the line.
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Let's repeat for x = 1
y = 2x+3
y = 2(1)+3 ... replace x with 1
y = 2+3
y = 5
So (1,5) is another point on the line
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We can keep this process going to generate as many points as we want. But to graph a linear equation like this, we only need two points at minimum.
Graph both (0,3) and (1,5). Then draw a straight line through them both. The graph is shown below.