You are given that 1 inch = 2.54 centimeters, so to convert 12.4 inches to centimeters, you would multiply 12.4 in by 2.54 cm.
You can answer this problem using the midpoint formula, but we can work out the 4th vertex by just logically thinking it through by first drawing out the rhombus on an x-y coordinate plane. I wrote the answer out on paper since it is easier than typing. Let me know if you have any questions.
Go here - http://www.purplemath.com/modules/rtnldefs2.htm
Answer:

it's was vertical angel they are equal ( = ) each other
it's was helpful to you