You have to do the opposite operations to get the variable on one side of the equation by itself.
4 - x + 4 = 8 - 4
x = 4
You only switch the comparison sign if you mutliply or divide by negative numbers when you do opposite operations to get the variable on one side of the equation by itself.
This shows that if you study 1 hour you get a 20. So to find the answer you multiply each side by 4.5, this means that if you study 4.5 hours you get a 90.
