<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def evaluate(self, flux, xo, yo, a, b, c):
 """ Evaluate the Gaussian model Parameters flux : tf.Variable xo, yo : tf.Variable, tf.Variable Center coordiantes of the Gaussian. a, b, c : tf.Variable, tf.Variable Parameters that control the rotation angle and the stretch along the major axis of the Gaussian, such that the matrix M = [a b ; b c] is positive-definite. References https://en.wikipedia.org/wiki/Gaussian_function#Two-dimensional_Gaussian_function """

        dx = self.x - xo
        dy = self.y - yo
        psf = tf.exp(-(a * dx ** 2 + 2 * b * dx * dy + c * dy ** 2))
        psf_sum = tf.reduce_sum(psf)
        return flux * psf / psf_sum