This is what is called the Midsegment Theorem, which states that the relation of a triangle's midpunkt is parallel to the triangle's third side, and the mid-segment length is half the third side length, so you would take half of and set that expression equal to the midsegment:
4x + 2 = 2x + 22
-4x-4x
____________
2 = −2x + 22
-22-22
___________
You then plug this back into both expressions above to get the double-segment of 84 and the mid-segment of 42. We can tell this is correct because 42 and 84 are relatively proportional to each other.