The line passes through (0, -3)
Its perpendicular to another line whose slope is 2
The slope of the line therefore is -1/2 = -0.5 (the product of slopes two perpendicular lines is -1)
Taking another point (x,y) on the line;
Slope = change in y-axis ÷ change in x-axis
-0.5 = (y - -3) / (x - 0)
y = -0.5x - 3