Answer:

the unknown number is 5.

[tex] SOLUTION;[/tex]

Let's denote the unknown number as "x".

According to the given information:

(x + 2) * 3 = 21

To find the value of "x," we can solve this equation.

First, let's simplify the equation:

3(x + 2) = 21

Now, distribute the 3:

3x + 6 = 21

Next, isolate "x" by subtracting 6 from both sides:

3x = 21 - 6

3x = 15

Finally, divide both sides by 3 to solve for "x":

x = 15 / 3

x = 5

Therefore, the unknown number is 5.