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 $\alpha$ .