Hey there!
Let's start by making an equation to represent this.
500 - 3a = 266
Reduced by is the same as subtracting, and a represents age.
Subtract 500 from both sides.
-3a = -234
Divide each side by -3.
a = 78
Check.
500 - 3(78) = 266
500 - 234 = 266
266 = 266
You are 78 years old.
Hope this helps!