Simplify the complex fraction.

x plus open parentheses four x over y closed parentheses over seven over three x

To simplify the complex fraction, first divide "x" by "y" which gives us:

$\frac{x + \frac{4x}{y}}{\frac{7}{3x}}$

Now, multiply the numerator by the reciprocal of the denominator:

$(x + \frac{4x}{y}) \cdot \frac{3x}{7}$

Now, simplify the expression:

$\frac{3x^2 + 12x}{7y}$

So, the simplified complex fraction is $\frac{3x^2 + 12x}{7y}$.

Can you please use math symbols in your answer?

Sure! Here is the simplification of the complex fraction using math symbols:

Given: $\frac{x + \frac{4x}{y}}{\frac{7}{3x}}$

Simplify: $(x + \frac{4x}{y}) \times \frac{3x}{7}$

Simplify further: $\frac{3x^{2} + 12x}{7y}$

Therefore, the simplified complex fraction is $\frac{3x^{2} + 12x}{7y}$.