y2-y1 divided by x2-x1

                                  2   y|                        1

           |

           |

            --------|origin---------x
           |

                       3     |               4

D=x values

R=y values

P= want over total

O= want over what you dont want

A: sum

S: difference, reduce by

M: product, triple

D: quotient, half, split into equal parts

no solution=infinite solutions ex. -4x=4x

zero=ex. 0=0

Zero= 0 on bottom

No slope= 0 on top

F: only has one output with every input

R: has more than one output for one input