Question

What is the equation of a line that passes through (7, 8) and has a slope of -3?

Answer 1

y=mx+b

M=slope
x= x coordinate
y= y coordinate

Plug in your numbers,
Solve for b
Then replace the x and y coordinates with the variables x and y.

:D

Answer 2

Alternately, lines have an equation of form called "point-slope form"\[y-y_0 =m(x-x_0)\] which is handy because you can plug in your point (x_0 and y_0) and slope, then manipulate to whichever form is needed