Answer:
9
Step-by-step explanation:
It is stating that 81 is equal to x², in which the variable x stands for a number.
Note the equal sign, what you do to one side, you do to the other. Isolate the variable, x, by rooting both sides:
√x² = √81
x = √81 = √(9 * 9) = 9
9 is your missing base.
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