Write an equation of the line that passes through the point (3,0) and
has the slope, m = -2.

The equation of a line is Y=mx+b
You know that the slope is -2 and the slope is 'm' so sub it into the equation.
Y=-2x+b
To have an equation of a line you need the 'b' or 'y' intercept and the slope.
This means you have to find the 'b' because you already know the slope which is -2. To find the 'b' you have to sub in the coordinates and then solve for 'b'
Y=-2x+b
(x,y)
(3,0)
0=-2(3)+b
0=-6+b
6=b
Therefore the equation of the line is Y=-2x+6