How old is Ana?

the phrase "Marvin is three times as old as Ana was..." means that the age of Marvin now is three times the age of Ana $x$ $years$ $ago$

the phrase "when Marvin was as old as Ana is now" means the age of Marvin $x$ $years$ $ago$ is equal to the age of Ana now

let: $M$ be the age of Marvin now;

$A$ be the age of Ana now;

$A-x$ be the age of Ana $x$ $years$ $ago$ ;

$M-x$ be the age of Marvin $x$ $years$ $ago$

$M=33=3(A-x)$

$33=3(A-x)$

$11=A-x$ ---->(1)

$M-x=A$

$33-x=A$ --->(2)

from (1), $x=A-11$

from (2), $33-A=x$

since the difference of their ages is always equal, we equate $x=x$ ,

$A-11=33-A$

$A=22$