How do you define a variable and write an expression for each phrase: the sum of 13 and twice a number?
2x + 13
The reason for this is because, well, you kinda gotta go backwards. Twice an unknown number, and unknown number being a variable (by definition) would leave it as x, and twice of x is 2x.
And to finish off, since it's the sum with 13, and since sum means addition, it would end it with 2x + 13.
A variable is something that can change and/or is unknown.
sum = + or addition
a number = a variable or unknown value.
13 = 13
(2n) = twice the unknown number
the sum of thirteen and w twice a number