You combine like terms by variable(s) and exponent(s).
The variables and exponent must match in order to combine.
So take #x^2 + x#. Variables (x and x) match, but exponents (2 and 1) do not, so you cannot combine
Take #x^2 + y^2#. Exponents (2 and 2) match, but variables (x and y) do not.
Now, take #3x^2 + 5x^2# These CAN be combine since the variable of x and power of 2 match.
So we have #3x^2 + 5x^2 = (3 + 5)x^2 = 8x^2#
Like Term Calculator