The missing variable x is 9.5.
To find the missing variable, you can set up an equation based on the given information and then solve for the variable.
In this case, you have the expressions 3x, 50, and 7x + 12.
You can set up the equation as follows:
3x+50=7x+12
Now, you want to isolate the variable x.
Start by combining like terms on both sides of the equation:
50=4x+12
Next, subtract 12 from both sides:
38=4x
Finally, divide both sides by 4 to solve for x:
x=

Simplifying the fraction, you get:
x=9.5
So, the missing variable x is 9.5.