The expression [tex]x^2+3[/tex] means 1) square x, then 2) add 3. You apply inverse operations in REVERSE order.
1. Subtract 3
2. "unsquare" [tex]x^2[/tex]
[tex]x^2+3=12 \\ x^2=9 \\ x=\pm 3[/tex]
The last step has two solutions in it, positive 3 and negative 3. That's because "unsquaring" really means take the square root, and 9 has two square roots, 3 and -3.
