Ok, so,, if the p=50, it means that l+l+w+w=50. (l is length and w is width.)
In simpler form, 2xl+2xw=50. In this case, we know that the width is 10. So,
2xl + 2 x 10= 50
2 x l +20=50.
Now we subtract 20 on both sides to get l alone.
2 x l=30
length= 30 divided by 2, or 15
:-)