Answer:
y = -3
Step-by-step explanation:
Each side of an equilateral triangle will have a length that is 1/3 the perimeter. This triangle has sides of length 21/3 = 7, so ...
y + 10 = 7
y = 7 - 10
This is consistent with the other given side measure ...
y^2 -2 = (-3)^2 -2 = 9 -2 = 7
5:36
12 inches = 1 foot, so 6 feet = 72 inches
10:72, reduces to 5:36