Answer
$3.6\;\rm cm,\;Concave$
Work Step by Step
We know for a spherical mirror that
$$\dfrac{1}{s}+\dfrac{1}{s'}=\dfrac{1}{f}\tag 1$$
and we know that the magnification is given by
$$m=\dfrac{-s'}{s}$$
Hence,
$$s'=-ms$$
Plugging into (1),
$$\dfrac{1}{s}+\dfrac{1}{-ms}=\dfrac{1}{f} $$
Solving for $f$,
$$f=\left[\dfrac{1}{s}+\dfrac{1}{-ms}\right]^{-1}$$
Plugging the known;
$$f=\left[\dfrac{1}{1.2}+\dfrac{1}{-(1.5)(1.2)}\right]^{-1}$$
$$f=\color{red}{\bf 3.6}\;\rm cm$$
Since the focal length is positive, so the mirror is concave toward the teeth.
