you spent $10.90 this week. This is $3.65 more than you.
If x represents the amount of money you spent last week, then, you have:
x + 3.65 = 10.90
the amount of money you spent last week plus $3.65 is equal to the amount of money you spent this week $10.90.
To solve the previous equation for x, subtract 3.65 both side of the equation:
x + 3.65 - 3.65 = 10.90 - 3.65 simplify both sides
x = 7.25
Hence, the amount of money you spent last week was $7.25