On his first three tests, hiram got scores of 89, 95, and 90. his final test score brought his median score to 91.5. what was hiram's final test score?

Median is one of the measures of center of data points along with mean and mode. This refers to the number of value which appeared in the center when the values are arranged in increasing order. If there are even number of data points, this is the mean of the two centermost values. 
 
In this case, if we are to arrange the values, we have,
  
     89, 90, x, 95

x is the score in the final test. that is placed in between 90 and 95 considering that the median is between these two numbers.

       91.5 = (90 + x) / 2

The value of x from the equation is 93.

The score in the third test is therefore equal to 93.