How can i write an explanation of how place value strategies your answer

Which of the choices shown could be used to prove that aacp=abcp ?

Two cyclists 84 miles apart start riding toward each other at the samen time. one cycles 2 times as fast as the other. if they meet 4 hours later what is the speed (in miles) of the faster cyclists

Find the area of an equilateral triangle with a side of 6 inches