Ok I managed to figure out the first part of this question on my own but the second part I just can't seem to wrap my brain around how to put the words into a solvable equation.

Here's the first part including the method I used to get the right answer:

Course Guide - To Get an A in a course, you must have an average of at least 90 points for four tests of 100 points each. For the first three tests, your scores are 87, 92, and 84. What must you score on the fourth exam to earn a 90% average for the course.

Answer is:

87 + 92 + 84 + x/4 = 90

263 + x/4 = 90

multiply both sides by 4

which will leave me with...

263 + x = 360

minus 263 from both sides and the it becomes

x = 97 (which is the answer in the book)

Now part two I just can't get. Please see the question below and if you are able to help that would be great.

Repeat excercise 75 if the fourth test is weighted so that it counts for twice as much as the first three tests.