6 plus twice the sum of a number and 7.

6 plus twice the sum of a number and 7.

The phrase a number mean an arbitrary variable, let's call it x.

The sum of a number and 7 means we add 7 to the variable x.
x + 7

Twice the sum means we multiply the sum by 2:
2(x + 7)

6 plus means we add 6 to 2(x + 7)
6 + 2(x + 7)