That number would be -3.
For the purpose of solving this, let that number be x. Now write out the front side of the equation:
2x (twice a number) - 5 (five less than)
2x - 5
Now write out the other side
1/2 (one half of) (3x - 13) (The difference of 3 times the number and 13)
1/2(3x - 13)
Now set the two sides equal to each other and solve
2x - 5 = 1/2(3x - 13) ----> multiply both sides by 2
4x - 10 = 3x - 13 ----> subtract 3x from both sides
x - 10 = -13 -----> add 10 to both sides
x = -3