The image shows a parallelogram, and the task is to find the value of $x$. In a parallelogram, opposite sides are equal in length. The given equation for one side is $3x - 15$, and the other side is 42.

To solve for $x$, set the two sides equal to each other:

$3x - 15 = 42$

Now, solve for $x$:

1. Add 15 to both sides: $3x = 42 + 15$ $3x = 57$

2. Divide both sides by 3: $x = \frac{57}{3}$ $x = 19$

Thus, the value of $x$ is 19.

Would you like more details on how I arrived at this solution, or do you have any other questions about parallelograms?

Here are some related questions to explore further:

1. How do you calculate the area of a parallelogram?
2. What is the difference between a parallelogram and a rectangle?
3. How do you find the perimeter of a parallelogram?
4. What happens to the area if the sides of the parallelogram are changed?
5. What is the formula for the diagonal length of a parallelogram?

Tip: Always remember that in a parallelogram, opposite sides are not only parallel but also equal in length.