Astudent took 5 tests this marking period and had a mean score of 90. her scores on the first 4 tests were 90, 96, 86, and 92. what was her score on the fifth test?

You can always treat such examples as an equation;
(90+96+86+92+X)/5=90
X is the desired score, 5 is the number of tests. As you can see, it is the usual equation for getting the mean value which the sum of the values over their number.
now; 90+96+86+92+X=90*5
X=90*5-(90+96+86+92)
X=86.
Hope this helps.