What is the degree of a polynomial equation is based on?

The highest power of 'x' in the equation.

Ax^(2) + Bx + C = 0 is of degree - 2

AX^(3) + BX^(2) + Cx + D = 0 is of degree - '3'.

et seq.,

The ' A B C D ' are numeral coefficients, and do not bear on the degree of the equation.