Final answer:
The phrase 'five more than three times a number equals 21' translates to the algebraic equation 3x + 5 = 21, which corresponds to option (a).
Step-by-step explanation:
To write the phrase 'five more than three times a number equals 21' as an equation, we need to translate the words into mathematical symbols. 'Three times a number' suggests multiplying a number, let's call it 'x', by 3, which is written as 3x. 'Five more than' indicates that we add 5 to this expression, thus we have 3x + 5. 'Equals 21' tells us that the result of this expression is equal to 21. So putting it all together, the equation we get is 3x + 5 = 21. The correct option that represents this equation is (a) 3x + 5 = 21.