Final answer:
The expression -4w^8 + 15a is already in its simplest form since it consists of two dissimilar terms that cannot be combined or further simplified.
Step-by-step explanation:
To simplify the expression -4w8 + 15a, we need to recognize that it is already in its simplest form because it contains two dissimilar terms - one with a variable w to the power of 8 and another with a variable a. Since these terms are not like terms, they cannot be combined or further simplified. Simplifying expressions is a key concept in algebra that involves reducing expressions to their simplest form by performing operations and combining like terms when possible.