I need to make a recursive equation with the table, that fits inside the blanks

Given:
[tex]\begin{gathered} Q_1=3 \\ Q_3=8 \\ Q_7=18 \end{gathered}[/tex]The recursive formula is:
[tex]Q_n=Q_{n-1}+2.5[/tex]For n = 2, 3, 4, ...
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