The general formula of a line in slope-intercept form is y = mx + n. In this formula m (and therefore the number in front of the variable x) represents the slope of the line.
We have the following formula in the question: y = x – 3. To find its slope we must look at the number in front of the x (as I already mentioned). Since there is no visible number we must realise that the number in front of x is 1. Therefore, the slope of the line is 1.