I just need to understand how its done. I don't understand the midpoint formula.

The midpoint formula is: [(x_{1} + x_{2})/2 , (y_{1} + y_{2)}/2)]_{
}A data point is always given in the form of (x, y). In your question, 12, 12 would be the data point at x = 12 and y = 12 as well as x = 2 and y = 12. These are 2 different points on a graph.

To solve your equation by using the midpoint formula, you must take each x from the data points given to you (12 and 2) and add them together. After adding them together, divide that number by 2. Then, take your y data points of 12 and 12, add them together,
and divide that number by 2. This will give you the midpoint of your 2 data points on a graph.

I hope this was helpful.