Which number is a constant in the algebraic expression -x^2-6y+13x+7

Let's take a close look at x^2-6y+13x+7.  Add the coefficient '1' in front of the first term:  1x^2-6y+13x+7.

The numerals {1, -6} are called "coefficients."
The final numeral, 7, stands alone.  This is the "constant."