The given equation seems incomplete. You likely meant to write:

$3 + x = 15$

If that's the case, I can solve it as follows:

$3 + x = 15$

Subtract 3 from both sides:

$x = 15 - 3$

$x = 12$

So, $x = 12$.

Let me know if you meant something different! Do you need more details on the steps or clarification?

Here are five related questions:

1. How would you solve $4 + x = 20$?
2. What happens if you add a negative number to both sides of an equation?
3. How do you solve $3 + 2x = 15$?
4. Can you solve $3 - x = 15$?
5. How do you solve equations involving fractions?

Tip: When solving linear equations, always perform the same operation on both sides to maintain balance!