Step 1: Substitute a = -2 and simplify the quadratic equation.
2.5 = -2t2+3t+1
Step 2: Find the discriminant of the equation and show that it is < 0.
The ball did not reach a height of 2.5 m. Therefore the flight path of the ball did not intersect the imaginary line 2.5m. Therefore the discriminant will be b2-4ac <0.
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