A term is a product of numbers and letter-variables

The degree in x of a term is the highest power of x in that term

The degree in x of a polynomial (which is a sum of terms) is the degree in x of its highest degree term.
Check the degree of each term.
Note the biggest of these degrees.
The degree of the polynomial is that biggest number.

In our case, the degree of "2x3 + 5x - 7" is 3