Question

Please i need help with this asap

Answer 1

`\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 = \frac{72}6 =12\\\\\text{Sum of finite arithmetic series,}\\\\S_n= \frac n2 (a+l) = \frac{12} 2 (3+69) = 6* 72 = 432`