To solve this system of equations, we can set the two expressions equal to each other:

3x + 7 = 6x + 4

Now, we can isolate the x variable by moving all terms with x to one side of the equation:

Subtract 3x from both sides:

7 = 3x + 4

Subtract 4 from both sides:

3 = 3x

Divide by 3 on both sides:

x = 1

Now that we have found the value of x, we can substitute it back into either of the original equations to find the value of the missing variable:

3(1) + 7 = 3 + 7 = 10

Therefore, the missing value is 10. The completed equation is:

3x + 7 = 6x + 4 = 10