How would I write an equation of a line with an undefined slope that passes through (-3,-6)?

Answer 1

undefined slope means the line is vertical

the equation is of the form ... x = ?

Answer 2

to have an undefined slope, the line must be a vertical line.
A vertical line has the form x = c, where c is a constant

look at our point, its x value is -3

so the equation of your line is x = -3

Answer 3

the equation is X= -3 hence the form of undefined equations is x=c