Square root of 121 is 11.
11-7=4
A side of the original square is 4 feet.
This is how you solve this problem:
.10d + .05n = 5.25
(plug in the number of dimes we have)
(21 x .1) + .05n = 5.25
(multiply number of dimes x value of the dime)
2.10 + .05n = 5.25
(subtract 2.10 from both sides)
.05n = 3.15
(divide both sides by .05 to get the number of nickels)
n = 63 nickels
Hope this helps!