Question

I don’t understand it when it’s reflected in the same axis

Answer 1

we have that

The rule of the reflection over the y-axis is equal to

(x,y) -----> (-x,y)

so

f(x)---------> g(x)

Apply the rule

f(x) ------> (-x,f(x))

g(x)=f(-x)

the answer is

g(x)=f(-x)