Write the equation of the line that 
has a slope of negative 2 and passes 
through the origin

Use

y = mx + b

m = -2 because that's what you are given, 
and the y-intercept is 0 since it passes 
through the origin (0,0), so just plug in 
m = -2 and b = 0

y = -2x + 0

y = -2x

Edwin