Step by step

Create an XY data table. There is one X column, and two Y columns. If the X values don't match for the two data sets, just leave some Y values blank, staggering the entry down the page.

Click Analyze, choose nonlinear regression, choose the panel equations for lines, and choose Two intersection lines --fit the crossing point.

Model

Y= Ycross + (X - Xcross)*Slope

Interpret the parameters

•Slope for the first data set

•Slope for the second data set

•Xcross. The X coordinate of the crossing point (shared for both data sets)

•Ycross. The Y coordinate of the crossing point (shared for both data sets).

That makes sense. You have two lines, and it takes two parameters (usually a slope and intercept) to describe each. Here, the equations were rearranged so the program doesn't fit two separate Y intercepts, but rather fits the X and Y values of the crossing point as well as two slopes.

Here is an example, with the key results circled. Prism file.