This is an excellent introduction to the topic of simultaneous equations.

http://www.heymath.com/main/samples/us19/teacherstemplate.html

Three possible models present themselves: two substitution methods and one elimination. The first and last allow equations to be entered in any form.

The second requires these to be entered in "y =" form - but they can be entered from either spreadsheet or graph.
http://compasstech.com.au/TNSINTRO/TI-NspireCD/mystuff/Dynamic_Algebra_S...