Verified answer

[tex]\huge \color{blue}\colorbox{white}{ solution : }[/tex]

[tex]a _{1} = 4 \\ Sn = na_{1} + \frac{n(n - 1)}{2} d \\ S_{9} = 9 \times 4 + \frac{9(9 \times 8)}{2} d \\ = 36 + 36d[/tex]

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[tex]S_{9} = 216 \\ \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: 36 + 36d = 216 \\ \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: 36d = 180 \\ \: \: \color{light}\colorbox{pink}{ answer : \: d = 5}[/tex]

[tex]\color{light}\colorbox{lightblue}{ common \: difference \: = 5}[/tex]