We have indeterminateness of type
0/0,
i.e. limit for the numerator is
$$\lim_{x \to a^+}\left(- \tan{\left(a \right)} + \tan{\left(x \right)}\right) = 0$$
and limit for the denominator is
$$\lim_{x \to a^+}\left(- a + x\right) = 0$$
Let's take derivatives of the numerator and denominator until we eliminate indeterninateness.
$$\lim_{x \to a^+}\left(\frac{- \tan{\left(a \right)} + \tan{\left(x \right)}}{- a + x}\right)$$
=
$$\lim_{x \to a^+}\left(\frac{\frac{\partial}{\partial x} \left(- \tan{\left(a \right)} + \tan{\left(x \right)}\right)}{\frac{\partial}{\partial x} \left(- a + x\right)}\right)$$
=
$$\lim_{x \to a^+}\left(\tan^{2}{\left(x \right)} + 1\right)$$
=
$$\lim_{x \to a^+}\left(\tan^{2}{\left(x \right)} + 1\right)$$
=
$$\tan^{2}{\left(a \right)} + 1$$
It can be seen that we have applied Lopital's rule (we have taken derivatives with respect to the numerator and denominator) 1 time(s)