Answer:
3 * (x + 7)
Step-by-step explanation:
Mark number as x
Then the sum of a number and seven is x + 7
Three times x + 7 can be written as
3 * (x + 7)
Parentheses mean that we first add x and 7 and then multiply their sum by 3
Alternatively you can write it as 3x + 21, since
3 * (x + 7) = 3 * x + 3 * 7 = 3x + 21
