The plot function creates a 2D plot with the slope-intercept form of the equation, and the (x,-10,10) part specifies the range of the graph, between the x values of -10 and 10. This will also be ...

That means you can refer to x with y = x * 10 in a cell above the cell that defines x. And, you can use a custom function at the top of your notebook but define it at the very end.