No. You insert X values and use the formula (solved for Y) for the corresponding Y values.

"Can't Excel solve them itself?" you ask. In a manner of speaking, it can. Put in a column of X values and a column of "guesses" for the Ys. Put in a third column from your function which should equal zero in all cases; e.g., Y + 2X - 3 in your first equation. Put in a fourth column which is the square of the values in the third column. Sum the values in the fourth column. Use Excel's solver (Tools, Solver) to minimize this last value by varying the Ys; this explains why you use the squares because the minimum will occur when they're all zero.

Once the solver has done its thing, the first two columns will have Xs and Ys to plot.

This seems like a lot of work when the algebra to solve a linear equation for Y is so simple, but if algebra should terrify you . . . .

PS I just tried this with your firse equation. I used X values from -10 to 10 in steps of 0.1 (201 values). I put all the Y's as zero. When I tried to run the solver telling it to vary all of the Ys it said I had too many variables. So I only used the first 101 (-10.0 to 0.0), then used the last 100 (0.1 to 10.0). It worked like a charm.