Solve for x: x²/4 - 3 = 6.

Add $3$ to both sides: $\frac{x^2}{4}=9$. Multiply by $4$: $x^2=36$. Take the square root, remembering both signs: $x=\pm6$, choice D. Check: $\frac{36}{4}-3=9-3=6$ for both $6$ and $-6$. Choice B ($6$ only) gives only the positive root. Choice A ($\pm3$) comes from taking the square root of $9$ before multiplying by $4$. Choice C ($\pm36$) forgets to take the square root of $36$. An equation of the form $x^2=k$ with $k>0$ always has two solutions.