[tex]\text{First term,} ~a = 3\\ \\\text{Last term,}~ l = 69\\\\\text{Common difference,} ~d= 9-3 =6\\\\\\\text{Number of terms, n =?}\\\\~~~~~~~l=a+(n-1)d\\\\\implies 69=3+(n-1)6 \\\\\implies 69 = 6n-3\\\\\implies 6n = 72\\\\\implies n = \dfrac{72}6 =12\\\\\text{Sum of finite arithmetic series,}\\\\S_n= \dfrac n2 (a+l) = \dfrac{12} 2 (3+69) = 6\times 72 = 432[/tex]
