Your code returns
j =
(delta^2*t^3)/3 + (eta^2*t^3)/3 + t
With this information, the symbolic portion of your problem is done. Now use standard MATLAB functions.
Create a function of x = [delta eta] that you want to maximize, and create appropriate constraints. For details, see Solve a Constrained Nonlinear Problem, Solver-Based, Writing Scalar Objective Functions, Maximizing an Objective, and Write Constraints.
Good luck,
Alan Weiss
MATLAB mathematical toolbox documentation