Answer
$y=f(x+3)$
Refer to the green graph below.
Work Step by Step
The given function can be written as:
$$y=\left[x-(-3)\right]^2$$
Recall:
The graph of the function $y=f(x-h)$ involves a horizontal translation ($h$ units to the right when $h\gt0$, $|h|$ units to the left when $h\lt0$) of the parent function $f(x)$.
The parent of the given function is $y=x^2$.
The given function can be written as $y=f(x+3)$.
With $h=-3$, the graph of the given function involves a $3$-unit shift to the left of the parent function $f(x)=x^2$.
Thus, the given function can be graphed by translating the parent function's graph $3$ units to the left.
Refer to the graph above.
The black graph is $f(x)=x^2$.
The green graph is $y=(x+3)^2$.