I would guess -6 because:

We are given the x-intercept = 4 and that will give you one side of the triangle, the side along the x-axis. We also know the area = 12, so just solve for the other side.

For triangles: 1/2 * base * height = area
1/2 * b * 4 = 12
b = 6

The only thing to remember now is that the slope of the line was positive, so we know that the y-intercept will be a negative number, and that gives us our answer of -6

OA is -6.

The problem is to solve this in a minute and 50 seconds.

I think the trickiest part of that problem is remembering that the slope is positive. Drawing out the problem definitely helped me as I just did it. As long as you remember 1/2BH=Area it can be done in less than a minute.

Best of Luck!