Write an equation for the line in the given form. Slope is - 2 and (3,9) is on the line; standard form

Standard form? That looks like either Ax + By + C = 0 or Ax + By = C.

I'll use the slope-intercept form first, and then convert my result to Standard Form.

y = mx + b, so here 9 = - 2 (3) + b = > 9 + 6 = b = 15, so

y = - 2x + 15. Subtracting y from both sides, we get 0 = - 2x - 1y + 15,

or 2x + 1y = 15 (which has the form Ax + By + C).