When looking for factors, you want to first look at the first and last terms.
To get x^2, the only possibilities are x * x.
For 64 there are many more factors, 1*64, 2*32, 4*16 and 8*8 and each of these containing two negative numbers.
The next thing you would want to do is find one of the factors that would sum to the middle number. In this case we are looking for two numbers that would add to 16. We can automatically get rid of any factors with negative numbers. The only factors that add to 16 are 8 + 8.
The factors are (x+8)(x+8) or (x + 8)^2.
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