This is for 30cm not 48 but you can find out how to do it. This is an example
Plug in 300 for C(x), to get:
300=

Then subtract to get: 0=

Then use the quadratic formula and plug everything in. Use a calculator to simplify.
You'll get two answers: 29.486 and -0.513.
Since you can't sell <em>"negative" </em>tacos, use the positive solution
Around 29-30 tacos.
Slope is your 2x, so 2x over 1 meaning after you plot (-2,2) on the graph you rise 2 and go 1 to the right, or do the opposite to make the line so go down 2 and one to the left. Hope that helped.