Answer:
a=-6, b=18
Explanation:
The midpoint can be defined as:

The order doesn't matter since addition is associative, just make sure to put them in the right place
So we know what the midpoint is, we can plug in known values to solve for the missing end point.
Let's start with the x-value

Multiply both sides by 2

Subtract 2 from both sides

Now let's do the y-value

Multiply both sides by 2

Add 6 to both sides

The point is at (-6, 18)