During the week Edward spent $90 (-90 from total amount). He has $45 remaining. Let x be the amount he started with
x - 90 = 45 is your equation.
To solve for x, isolate it. Note the equal sign. What you do to one side, you do to the other. Add 90 to both sides
x - 90 (+90) = 45 (+90)
x = 45 + 90
Simplify
x = 135
$135 is the amount Edward started out with in the beginning of the week.
hope this helps
