Anti-Gravity in projectiles

Classical Mechanics Level 5

In the vertical xy plane a particle is projected with a velocity \( 10 m/s \) making an angle \( { 45 }^{ 0 } \) with the \( +ve \) x-axis. Acceleration due to gravity is \( -10j m/{ s }^{ 2 } \).

At time \( t=\alpha \) sec the anti-gravity switch is turned on. Now the acceleration is \( 10j m/{ s }^{ 2 } \). If the particle passes through the point \( (30, 20) \) during it's motion then: Find \( \alpha \).