My 12" British Thornton still lives on my desk and gets used for occasional light relief or when somebody's borrowed my calculator.
As it happens, yes, the higher level Casios will solve quadratics and simultaneous equations - a facility that I probably use at least annually ;-) When I did some AFCS design work once I found the high end casio calculator's ability to manipulate 6x6 matirices particularly useful. With the best will in the world, and much as I love it, my slide rule just doesn't do that.
G