Answer
${\bf 2}\;\rm mm^{-1}$
Work Step by Step
We know that the area under the probability density curve is 1.
And we have here a right triangle which area is given by
$$A=\frac{1}{2}ba$$
where $b$ is the base length and $a$ is its height (usually we use $h$ for height but we used $a$ here to match the unknown).
Hence,
$$a=\dfrac{2A}{b}=\dfrac{2\times 1}{(2-1)}=\color{red}{\bf 2}\;\rm mm^{-1}$$
