Final answer:
The value of 3x is 15, derived by first solving for x in the equation 5x - 4 = 21 to find x = 5, and then multiplying by 3.
Step-by-step explanation:
The question asks to find the value of 3x given the equation 5x - 4 = 21. To solve for x, we first add 4 to both sides of the equation, yielding 5x = 25.
We then divide both sides by 5 to find x = 5.
Now that we have the value of x, we multiply it by 3 to find the value of 3x, which is 3*5 = 15.