Answer
Find the horizontal component of the initial velocity. The initial angle is 45 degrees and the initial speed was 141 m/s. The vertical and horizontal velocity components are equal.
$$v^{2}+v^{2} =(141 \frac{m}{s})^{2}$$
$$v=100 \frac{m}{s} $$
At the top of its flight, the vertical component of velocity is zero. The horizontal component does not change, and is still 100 m/s.
This is discussed on page 187.
