Hi Natakie.

Try using algebra. You have to re-write the question in terms of algebraic variables.

Let p = Petra's age and m = Mother's age.

Now interpret the question:

"the sum of the ages of petra and her mother is 51."

p + m = 51

"Her mother is 12 years more than twice as old as petra."

m = 12 + 2p

Since we know that m = 12 + 2p and p + m = 51, we can replace m in
one equation by what it equals in the other equation, so

p + m = 51 becomes p + (12 + 2p) = 51.

Can you solve this last equation and find Petra's age? Once you have that, you can easily determine the mother's age, too.

Hope this helps,
Stephen La Rocque.