we know that
1 ft is equal to 12 in
12 1/2 ft------> (12*2+1)/2-----> 25/2 ft
3 ft 7 in
7 in------> 7/12 ft
so
3 ft 7in-----> 3 (7/12) ft----> (3*12+7)/12-----> 43/12 ft
the small slide=(25/2)-(43/12)
multiply (25/2) by 6/6-----> 150/12
so
the small slide=(25/2)-(43/12)----> 150/12-43/12----> 107/12 ft
the answer is
the small slide is 107/12 ft long
