Write an equation of the line that passes through the point (3,40 and has a slope of -4

I assume the point is (3, 4).
The slope is -4

You can use the point=slope form of the equation of a line.

y - y1 = m(x - x1)

whee m = slope, and (x1, y1) is a point on the line.

You have m = -4, x1 = 3, y1 = 4

y - 4 = -4(x - 3)

In this problem it decreased 20%