Answer: 
Step-by-step explanation:
You need to use the following formula for calculate the area of a square:
Where "s" is the lenght of any side of the square.
You know that the area of a mural is 
. Then, you can substitute this area into the formula and solve for "s", in order to find the side lenght of the mural.
Therefore, this is:
Time used for the call to Zurich: ($5-$1.25)/$0.25=15 minutes
15minutes+3minutes=18minutes
Time used for the call to London: ($7.25-$1.25)/$0.25=24 minutes
24minutes+3minutes=27minutes
Difference between the two calls:27-18=9minutes
