When you add/subtract polynomials, you have to perform the said operation between like terms. Like terms are those that have the same variable (letter) and the same exponents, for example, 2x and 8x are like terms because they have the same variable or 9y²z and y²z, these are also like terms, they have the same variables y and z and both variables have the same exponents.
If both terms are like terms, then you have to perform the addition/subtraction between the coefficients of each term. (Remember: The coefficients are the values that multiply the variables.)
For the given addition
Both terms have the same variables and exponents "m³n", to add them you have to add their coefficients, which means that you have to add 5 and 29: