Right Triangles
The right triangle given in the figure has two given sides of 6 and 8 units respectively.
It's important to identify them as the hypotenuse or one of the legs.
As a rule, the longest side (or the side opposite to the right angle) is the hypotenuse, which measures 8 units.
The other two sides are the legs, one of which measures 6 units.
The required angle (marked as ?) is adjacent to the leg of 6 units.
The only trigonometric ratio that relates the adjacent side of a triangle with the hypotenuse of the triangle is the COSINE.
Now we calculate the measure of the required angle (we'll call it x):

The angle can be calculated by using the inverse cosine function:

Using a scientific calculator:
x = 41.41°
Rounding to the nearest degree:
x = 41°