Given that I know the value of a single point (x1,y1) on a line and its splope m, what formula(s) can I use to determine the two other points that are exactly d units from the known points.

This is for use in a server side web application, written in php.

At 60 yrs old, I have forgotten all that I thought I knew about doing this

That includes trying to solve for two variables in a set of equations, which I suspect this will need

TIA,