Here is just a start:
Clear[u, x, t, \[CapitalPhi]];
eq21 = D[u[x, t], t] + 5 D[u[x, t] D[u[x, t], {x, 2}], x] +
5 u[x, t]^2 D[u[x, t], x] + D[u[x, t], {x, 5}]
eq22 = u -> Function[{x, t}, 6 D[Log[\[CapitalPhi][x, t]], {x, 2}]]
subst = (eq21 /. eq22)/6;
eq23 = Integrate[subst, x] // Together // Numerator