Question 913547
p for speed of the PLANE if no wind
w for speed of wind entirely in one direction or the other direction


__________________rate__________time_________distance
DET to DEN________p-w____________8___________1120
DEN to DET________p+w____________7___________1120


The uniform rates travel rule is RT=D rate time distance.  Your data give two linear equation in the variables p and w.
{{{highlight(system((p-w)*8=1120,(p+w)*7=1120))}}}.
Solve the system.
This IS just as you have; which is two equation in two unknowns.  YES, substitute!  Do you prefer to use the elimination method?  You can use that if you want instead.


My impression is that you are only temporarily stuck on nothing.  You only <i>believe</i> you are stuck.