What is A squared plus 6 squared equals 12 squared?

a2 + 62 = 122

a2 + 62 - 62 = 122 - 62

a2 = 144 - 36

a2 = 108

taking the square root of each side, we get a equal plus or minus the square root of 108, or plus or minus 6 times the square root of 3.