keeping in mind that a year has 12 months, thus 18 months is really 18/12 of a year, so
[tex]~~~~~~ \textit{Simple Interest Earned Amount} \\\\ A=P(1+rt)\qquad \begin{cases} A=\textit{accumulated amount}\\ P=\textit{original amount deposited}\dotfill & \$4300\\ r=rate\to 5.1\%\to \frac{5.1}{100}\dotfill &0.051\\ t=years\to \frac{18}{12}\dotfill &\frac{3}{2} \end{cases} \\\\\\ A=4300[1+(0.051)(\frac{3}{2})] \implies A=4628.95[/tex]