$$5\ tan(x) + 3 = tan(x) \quad | -3$$
$$5 \ tan(x) = tan(x) - 3 \quad | - tan(x)$$
$$5 \ tan(x) - tan(x) = - 3 \quad | \ tan(x) \text{ausklammern}$$
$$tan(x) \cdot (5 - 1) = -3$$
$$ 4 \ tan(x) = -3 \quad | :4$$
$$tan(x) = - 3/4 \quad | \ arctan()$$
$$x = arctan(-3/4)$$