trigonometry – velocity and acceleration and their normality

The position vector of a particle at time t is $vec{r}= cos{(t-1)}hat{i} + sinh{(t-1)}hat{j} + at^3 hat{k}$

Find the condition imposed on a by requiring that at time t = 1, the acceleration is normal to the position vector.

How to answer this question? Any math help will be accepted.