I will assume you mean,
X2 - 16 = 0
X2 = 16
take square root each side
X = (+/-) 4
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(-4, 0) and (4, 0)
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-4 and 4
x = { +6, -6 }
x2 + 49 = 0 ∴ x2 = -49 ∴ x = 7i
x2≤64
If you meant x2 = 16 then take square root both sides which gives x = 4 but you need to see that -42 is also 16 so that is another solution. The two solutions are 4 and -4.