Final answer:
To make the ratios 3:9 and 6:___ equivalent, the missing number should be 18. The equivalent ratio is 6:18.
Step-by-step explanation:
To make the ratios 3:9 and 6:___ equivalent, we can use cross multiplication.
Cross multiplying means multiplying the numerator of one ratio with the denominator of the other ratio and vice versa. So, 3 * x = 6 * 9.
Simplifying the equation, we get: x = 18.
Therefore, the missing number is 18.
The equivalent ratio is 6:18.