The standard form of a polynomial is
f(x) = anxn +
an-1xn-1 + . . . + a1x + a0
Thus, the degree of the polynomial is n, which is the largest exponent which appears in the above polynomial.
3x has degree 1
(x3 + 5) has degree 3
(x2 + 2x) has degree 2
(x5 + 2x4 + 5x2 + 10x) has degree 5.