You are given the initial velocity 28 m/s. So first find out how much the velocity reduces after traveling 10 m high, in order to find your new initial velocity.

The new initial velocity, when the projectile is 10 m off the ground, is 24.2487. Then use another kinematic equation to solve for a normal projectile flight.

You want to know when the velocity is zero, and this will be half the time of the total flight (when the projectile is highest in the air). Solve for t, then multiply by 2 to get the final answer.