How should I solve this simple optimization problem? Is it related to optimal control?

I was wondering what are the correct FOCs for this problem:
$max_a int_{c_1}^{d_1} int_{c_2}^{d_1} f(a(theta_1,theta_2)) dtheta_1 dtheta_2$ with constraint $frac{partial a(theta_1,theta_2) }{partial theta_1}geq 0$ for all $theta_1 in (c_1,d_1),theta_2 in (c_2,d_2)$