John's mother is 5 times as old as John is. Three years ago, she was 9 times his age. What are their present ages?
Have a think.
Let John's age be $\displaystyle x$
His mother's age will then be $\displaystyle 5x$
So three years ago she was nine times his age...
$\displaystyle 5x-3 = 9(x-3) $
The $\displaystyle -3$ on the left side represents 3 years ago, you also have to do it on his mother's age. Since she is nine times the age just multiply the whole side by 9.