The problem asks us to find the expression that is equivalent to the given expression $5x + 6x$.

1. Problem Description

The problem asks us to find the expression that is equivalent to the given expression $5x + 6x$.

2. Solution Steps

We can simplify the given expression by combining like terms:

$5x + 6x$

Since both terms have the same variable $x$, we can add their coefficients:

$5 + 6 = 11$

So, $5x + 6x = 11x$.

Now, let's check the options:

* $6x + 6x = 12x$

* $x + 9x = 1x + 9x = 10x$

* $x \cdot 11 = 11x$

* $x^{11}$ is not a linear term and hence, not equivalent.

Thus, $5x + 6x = 11x$ is equivalent to $x \cdot 11 = 11x$.

3. Final Answer

$x \cdot 11$