x=15 and i have to write some more for 20 letters
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We need to solve for x, we need to get x alone
Lets start by removing -5
Add 5 on both sides
Now to isolate x , we need to remove the square from x
To remove square , take square root on both sides
square and square root will get cancelled
So 
and 
It is a very simple question to answer.
Obviously first we can calculate his weekly earnings and then multiply them by 4 to calculate his total earnings over the course of 4 year period.
So as he makes 6.2$ an hour. He will make 6.2 * 30 = 186 $ a week as that is the number of hours he works in a week.
Now we can multiply this by 4 to calculate his over all earnings. So 186 * 4 = 744.
