Where is the error: $$4\left[\left(12-6\right)\div2\right]^2$$

Step 1: $$\left[4+6\div2\right]^2$$

Step 2: $$\left[4+3\right]^2$$

Step 3: $$7^2$$

Step 4: $$49$$

Which of the following is not a function.

What is the center of the circle: $$f\left(x\right)=\left(x-4\right)^2+\left(y+5\right)^2=16$$

What is the radius of the circle: $$f\left(x\right)=\left(x-2\right)^2+\left(y-4\right)^2=36$$