algebra precalculus - Negative Squared Root on Quadratic Equation formula?

I have this basic problem:

In a farm, $X$ animals are added to the farm. These animals gain weight according to the equation: $500 - 2X$ gr. Which interval of animals can the farm take, if the total weight gain is greater than $30,600$ Kg?

Original (Spanish - Español):

En un criadero de cuyes se integran $x$ cuyes, si se tiene presente que los cuyes ganan peso en promedio de $(500 - 2x)$ gramos. ¿Qué intervalo de cuyes puede aceptar esta granja si la ganancia total de peso de los cuyes es mayor a $30 600$ Kg?

Step 1:

Total animals: $x$.

Weigthgain = $(500 - 2x)$ gr.

TotalWeightgain > $30 600(1000)$ converting kg to gr.

Step 2:

$$x(500 - 2x) > 30600000$$

Step 3:

$$0 > 2x^2 - 500x + 30 600 000$$

Step 4:

$$x = \frac{-(-500) +- \sqrt[2]{(-500)^2 - 4(2)(30 600 000)}}{2(2)}$$

But as you can see, the sqrt is negative. So It does not exist.
What should be the next step?