I would add 7 to get ...
... 2x² = 16
then divide by 2 to get
... x² = 8
Now, I would take the square root, recognizing that both positive and negative values will solve the problem.
... x = ±√8
The root can be simplified, so we have ...
... x = ±2√2
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The above method seemed the most obvious method to me. One could use the quadratic formula, but that seems to involve more operations.
... 2x² -16 = 0 . . . . subtract 9
... x = (-0 ± √(0² -4·2·(-16)))/(2·2) . . . . put the coefficients into the formula
... x = ±(√128)/4 = ±√8 = ±2√2 . . . . . simplify the result