Final answer:
Sam's expression 3√64 simplifies to 4, while Lisa's expression √84 simplifies to 2√21. Mark's sum of Sam and Lisa's answers is an expression, 4 + 2√21, because 2√21 cannot be simplified into a whole number.
Step-by-step explanation:
The question involves evaluating cube roots and square roots and then adding the results. Sam evaluated 3√64, which simplifies to 4, because 64 is 4 cubed (4 x 4 x 4 = 64). Lisa evaluated √84. We can simplify √84 by finding the largest perfect square that divides into 84, which is 4, so √84 = √(4 x 21) = 2√21. Since √21 cannot be simplified further, Lisa's result is 2√21. Mark added together Sam's answer (4) and Lisa's answer (2√21), but since 2√21 is not a whole number and no further simplification is given, we cannot find an exact numerical sum. Therefore, Mark's sum is an expression: 4 + 2√21.