When empty, a particular helicopter of mass 3770 kg can accelerate straight upward at a maximum acceleration of 1.42 m/s2. A careless crewman overloads the helicopter so that it is just unable to lift off. What is the mass of the cargo?

I tried dividing the mass by acceleration but that didn't work.

Of course it didn't work, why on Earth would you think it would?

ThrustEngine=(M+m)(g+a)

in the first sentence, m is zero. Figure thrust.
then, use the Thrust to compute m if a is zero.