Question

how do you graph the equation y=-2cosx?

Answer 1

By drawing a rectangular coordinate system, the axes labelled x and y respectively. Proceed further by make a table in which you calculate the value of y for given x. Use these values to draw the graph in the coordinate system, the x-value being the abscissa and the y-value the ordinate.
When drawing use the approximation pi = 3.

Answer 2

I do not recommend using a pi = 3 approximation. You will end up with large y errors for |x|>10 . You can make the plotting easier if you choose x table entries and graph x-axis divisions that are multiples of pi/6 (30 degrees).

Answer 3

Would also be a possibility, however, it does depend on the scale, as you surely know. If we set pi ^= 3, it is clear what is meant and the recipient can easily calculate how the function looks like in real.

Answer 4

Use Pi = 3 ???

reminds me of the alleged Indiana State Legislature bill which attempted to make it law that pi be a rational number.
http://www.cs.uwaterloo.ca/~alopez-o/math-faq/mathtext/node18.html

I also recommend drwls' method, of using a unit grid value to represent pi/6 or even pi/12 (15 degrees)
The pi/12 division gives us exact positions for the most popular angles such as pi/4 (45 degrees) etc.