We are given
length of rectangular yard is x
so, 
width of rectangular yard is
so, 
We are given
the length of the enclosed yard is 50 feet
so, we get

now, we can plug x=50 and then we can find width


now, we can find total fencing length
so, total fencing length is

now, we can plug values


now, we are given
the cost of fencing is $13 per foot
so,
total cost = (cost of fencing)*(total fencing length)
we can plug values


So, Janice will spend $4030.........Answer