I believe the answer is $33.60?
So we know that an average is the sum of the scores divided by the number of scores that were added together. In this case we are going to be dealing with 4 tests. So let's say x is the score for our fourth test. We can create this equation:

So then we need to simplify the top:

Then multiply by 4 on both sides to get rid of the denominator:

Then solve for x:

So now we know that Sam needs to score a 67 on the fourth test to have an average score of 50.
The sad thing is, I don't know either.
First you need to get two similar terms to eliminate, I'll do y:
-3x + 2y = 10
-4x - 2y= -10
And then you minus
x = 20
Now you can substitute x in to one of the equations to find y:
-60 + 2y = 10
+ 60
2y = 70
÷ 2
y = 35
So your values are y = 35 and x = 20. Hope this helps!