Explanation:
imagine the triangle is rotated so that it stands up, and the right angle is at the bottom right vertex.
what is that ?
or usual trigonometric triangle inscribed into an imaginary circle with the center at the vertex with the 68°.
and 10 is the radius of the circle and the 68° angle.
remember ?
sine is the up/down leg, cosine is the left/right leg of that triangle.
of course, since it is not the norm-circle, the radius is not 1, and we have to multiply every trigonometric function also by the radius (10).
so,
x = cos(68)×10 = 3.746065934... ≈ 3.7 units