The Trend Line $\tilde { y } _ { t }$

Essay

The trend line $\tilde { y } _ { t }$ = 125 + 2t and seasonal indexes shown in the table below were computed from 10 years of quarterly data.Forecast the values for the next four quarters.
$$\begin{array} { | c | c | } \hline \text { Quarter } &{ \mathbf { SI_t } } \\\hline 1 & 0.6 \\2 & 1.3 \\3 & 1.6 \\4 & 0.5 \\\hline\end{array}$$