For this case we must find the value of the following expression:

When:

Substituting the given values we have:

Finally, the value of the expression is 394.
Answer:
The value of the expression is 394.
The problem is 3+6x = 15 with x being the number of white numbers.
Isolate variable by subtracting 3 on both sides: 6x = 12.
Isolate variable by dividing 6 on both sides: x = 2
The number of white marbles is 2.