Two interpretations:
(1) Price of lot is L, price of house is H. H=9L is assumed to mean 9 times more than (as much as) the lot.
L=H/6-5000.
Substitute for H: L=9L/6-5000=3L/2-5000.
3L/2-L=5000.
L/2=5000 so L=10000 and H=9L=90000.
(2) Price of lot is L, price of house is H. H=9L+L=10L, is assumed to mean 9 times more than the lot.
L=H/6-5000.
Substitute for H: L=10L/6-5000=5L/3-5000.
5L/3-L=5000.
2L/3=5000 so L=3*5000/2=7500 and H=10L=75000.