How to create a softkey in equation lib to show a PNG that illustrate the equation?

[matalog]

I haven't looked at the library, but to make a softkey, you draw a rectangle on screen with RECT_P(blabla), then use TEXTOUT_P(blabla) to write in it.

You can make the area sensitive to touches on the touch screen with MOUSE testing if it returns numbers between the corners of the rectangle.

Finally you can use G0:=AFiles("image.png") to load an image, and you can keep track of the page you are on with a variable that updates on each page.

If you are unfamiliar with any of these terms or commands then you should look them up in the Programming section of the manual for the HP Prime available at - https://www.hpcalc.org/details/7445