M=undefined passing through p(-7,-4)

M=slope=change in y/change in x=rise/run

it can be undefined when the denomenator is equal to 0
meaning the run or change in x is 0
means the graph doesn't go left or right
means the graph is a vertical line
means the graph is y=c where c is a constant

see the y term
(-7,4)
(x,y)
y=4 is the equation

whenevery you have a slope that is undefined, the equation will be y=c
whenever  you have a 0 slope, the equation will be x=c