To simplify the expression \( 15^? \div 15^7 = 15^{12} \), you can use the property of exponents:
\[ \frac{a^m}{a^n} = a^{m-n} \]
Applying this property here gives:
\[ 15^? \div 15^7 = 15^{? - 7} \]
So we can rewrite the equation as:
\[ 15^{? - 7} = 15^{12} \]
Since the bases are the same, we can set the exponents equal to each other:
\[ ? - 7 = 12 \]
Now, solve for \( ? \):
\[ ? = 12 + 7 = 19 \]
Thus, the missing number is \( 19 \).
The answer is \( \boxed{19} \).