A special kind of triangle is a right triangle whose definite characteristic is having a 90° angle. It is also special because there are derived trigonometric formulas/identities that can be applied to it. A right triangle has three sides: hypotenuse, adjacent side and opposite side. The hypotenuse is the longest side of the right triangle. You can determine this visually, if not, quantitatively. The other sides are with respect to a given angle.
For the problem, it might have asked for the opposite side or the hypotenuse. We use the cosine function to find the hypotenuse:
cos 58° = adjacent/hypotenuse = 4/hypotenuse
hypotenuse = 7.55 units
Then, we use the tangent function to find the opposite side.
tan 58° = opposite/adjacent = opposite/4
opposite = 6.4 units