what is the equation of a line with a slope of -2 and passing through 0,3

The general form of a line is:

y = mx + c

Where m is the slope and c is the y-intercept.

What you need, is c, and you have already m, which is -2. So, use the point that you have. (x, y) = (0, 3), meaning that x = 0, and y = 3.

(3) = (-2)(0) + c

Find c, then put that in the general form of the equation of the line together with m.

What do you get? :)