There is a formula for factoring the "difference of squares." In this case, the answer is
(9 - 4x)(9 + 4x)
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9 x 9 = 81 9 squared = 81 3 squared times 3 squared = 81
x2-81=(x-9)(x+9) Factors: x-9, x+9
9 squared is 81. Two digits lower than 81 is 79. 79 is a prime number. Its only factors are one and itself. That does not compute. 78 has 26 and 6 as factors. 78 is three digits lower than 81. That does not compute.
16, 81, 625 and so on. Prime numbers to the 4th power.
Square numbers like 1, 4, 9, 16, 25, 36, 49, 64, 81, 100