See notes for lectures 3 and 4 in single pdf for Lecture 2. Here is a link to this surface plotter so you can have a bit of fun plotting some functions of two variables and looking at their graphs in 3d. https://academo.org/demos/3d-surface-plotter/ [I haven't tried using this on a mobile device; if you use your phone for doing maths and you know a better surface plotter app/website then please let me know.]