The expression in the image is:

$a = \sqrt{(3 - \sqrt{(6 - \sqrt{(10 + \sqrt{(15 + \sqrt{(21 - a)})})})})}$

This is an equation where $a$ is defined in terms of itself through a nested square root expression. This type of equation is quite complex due to the recursive nature of $a$. We can try to simplify or solve it by assuming a value for $a$ and verifying if it satisfies the equation, or we can attempt to find an analytical solution.

Solving the Equation:

To find a solution analytically, let's simplify the equation step by step.

1. Assume $a$ is a specific value and substitute it into the expression.
2. Simplify the expression and compare it to the assumed value.

However, solving this directly might be very complex analytically. A common approach to handle such equations involves iterative approximation