If you are trying to find the equation of a line:
The equation of a line is y = mx + b [m is the slope, b is the y-intercept or the y value when x = 0 ---> (0, y) or the point where the line crosses through the y-axis]
Since you have m = -2, plug it into the equation
y = mx + b
y = -2x + b To find b, plug in the point they gave you (3, -7)
-7 = -2(3) + b
-7 = -6 + b Add 6 on both sides
-1 = b Now that you have b, plug it into the equation
y = -2x - 1