A nomograph is a kind of analogue computer or graph that lets you calculate solutions to a equation graphically, with an arbitrary number of variables. They are essentially static slide rules with one equation "pre-programmed."  It's a little unclear what that means in practice, but this example should help: The dashed line is a solution, and the way these are constructed, any straight line is also a solution. So you can simply take the two variables you know, drop a straightedge