.
The gardener/the fence has 3 times the length L and 2 times the width W (it is the FULL wide, not its half).
So the equations are
3L + 2W = 100, (1)
L*W = 336. (2)
From (1), W =
. Substitute it into (2) replacing W. You will get
= 336, or
L*(100-3L) = 672,
100L - 3L^2 = 672,
3L^2 - 100L + 672 = 0.
=
=
.
=
= 24 --->
=
= 14.
=
= 
--->
=
= 36.
Answer. There are two solutions: 1) the rectangle has the outer dimensions 24 m and 14 m;
2) the rectangle has the outer dimensions 
m and 36 m.