A = B*L
A = 21/50 sq miles
B = 3/5 mile
L = ? mile
L = A/B
L = (21/50)/(3/5)
* dividing by fraction is same as multiplying by it's reciprocal
L = (21/50)*(5/3)
L = (21*5)/(50*3)
L = 105/150
*divide both numerator & denominator by 5
L = 21/30
*divide both numerator & denominator by 3
L = 7/10 miles