i am not sure of what is substitution but here is my guess of what it means
you've got your first equation
ax+by=c (general case ) it could be write as ax-c=-by
so y=(c-ax)/a (you should be familiar with that sort of manipulation, check a is non null)
so in your second equation
you replace the y by the expression find above in the second equation you should get an equation of the form ax=b (when compacted) so you know how to find x (if a is null then b must be null or no solution) then you find y using the expression you where ... using!
no needs to say that you must check you allway does in math, twice or more is beter