I believe the answer is 311.157055138. That is if i did my math correctly. I hope this helps. Good luck!
Let sister be s. Matthew, M
Matthew is 7years older than twice the age of his sister
M = 7 + 2s
13 = 7 + 2s. 7 crosses the equality to other side and becomes -7
13 - 7 = 2s
6 = 2s Divide both sides by 2
6/2 = 2s/2
3 = s
Sister is 3 years old.
If
then the derivative is
Critical points occur where 
. This happens for
In the first case, we find
In the second,
So all the critical points occur at multiples of 
, or 
. (This includes all the even multiples of .)
First is yes
Second is no
Third is no
Fourth is no
