If the eq of trajectory of a projectile is given, how to find the time of its flight. For eg if eq of trajectory is y=ax -bx2 where y is height n x is horizontal dist from the pt of projection, how to get d time of flight??

Consider @ as Theta

The motion of trajectory is y=xtan@ - (1* g*x²) / 2 * u²cos²@

So, x = ucos@ *t  -- eqn.(1)

also, from eqn(1), we get

x / ucos@ = t;

so in the equation of trajectory, x²/u²cos²@ = t²

putting t² in the equation of the trajectory we get,

y = xtan@ - (1/2 * g * t²)

also,  xtan@ = x* usin@/ucos@

solve like this u will get...