Solve[qa==24-ba*pa+pb  &&qb==24-bb*pb+pa, {pa,pb}]

ClearAll["Global`*"]

pia =(-((-24-24 bb+bb qa+qb)/(-1+ba bb)) -ca)qa
dpia = D[pia , qa]
Solve[dpia==0, qa]

fqa[ba_,ca_,bb_]:=((24+24 bb+ca-ba *bb  *ca-qb)/(2 bb))
ss= Solve[qa==fqa[2,3,2],qb]
 ss1=ss[[1]]
 qb1=qb/.  ss1

fqa2 =((24+24 bb+ca-ba *bb  *ca-qb)/(2 bb))
fqa2/. {bb->3,ba->3, ca->5}

plot1=Plot[qb1, {qa,0,20}, AxesOrigin->{0,0}]

fqb[ba_,cb_,bb_]:=((24+24 ba+cb-ba *bb *cb-qa)/(2 ba))
fqb[2,3,2]

plot2=Plot[fqb[2,3,2], {qa,0, 80}, AxesOrigin->{0,0}]

Show[plot1, plot2]

a1=Solve[x^2+5x+2==0,x]
 a2 =a1[[1]]
a3 = x/. a2
a3