Total vertical distance travelled by a projectile

A student throws a ball for his dog to retrieve. The position vector of the ball, relative to an origin $O$ at ground level $t$ seconds after release, is given by $\tvec{r}_B(t) = 5t\tvec{i} + 7t\tvec{j} + (15t - 4.9t^2 + 1.5)\tvec{k}$. Displacement components are measured in metres, where $\tvec{i}$ is a unit vector to the east, $\tvec{j}$ is a unit vector to the north and $\tvec{k}$ is a unit vector vertically up.

The total vertical distance, in metres, travelled by the ball before it hits the ground is closest to