From Mechanics

Here we use \displaystyle \mathbf{r}=\mathbf{u}t+\frac{1}{2}\mathbf{a}{{t}^{\ 2}}+{{\mathbf{r}}_{0}} with

\displaystyle {{\mathbf{r}}_{0}}=1\textrm{.}5\mathbf{j} \text{ m}

\displaystyle \mathbf{u}=15\mathbf{i}+18\mathbf{j}\text{ m}{{\text{s}}^{\text{-1}}}

\displaystyle \mathbf{a}=-10\mathbf{j}

\displaystyle t=3\text{ s}

Giving \displaystyle \mathbf{r}=(15\mathbf{i}+18\mathbf{j} ) \times 3+\frac{1}{2}(-10\mathbf{j}) \times {{3}^{\ 2}}+1\textrm{.}5\mathbf{j}

Simplifying \displaystyle \mathbf{r}=45\mathbf{i}+54\mathbf{j}-45\mathbf{j} +1\textrm{.}5\mathbf{j}=45\mathbf{i}+10\textrm{.}5\mathbf{j} \text{ m}