If a projectile is shot at an angle of (3pi)/8 and at a velocity of 21 m/s, when will it reach its maximum height?

The projectile will move upwards because of its vertical component of velocity i.e $21 \sin \left(\frac{3 \pi}{8}\right)$
Let,the projectile will reach it's maximum height in time $t$,so we can write, $0 = 21 \sin \left(\frac{3 \pi}{8}\right) - g t$ (using, $v = u - g t$ and at the highest point of motion velocity becomes zero)
So, $t = 1.98 s$