\(\left(1+4+7+...+100\right):a=17\)
\(a=17\times\left(1+4+7+...+100\right)\)
\(a=17\times\left[\frac{\left(100-4\right):3+1}{2}.\left(1+100\right)\right]\)
\(a=17\times\left(\frac{33}{2}.101\right)\)
\(a=17\times\left(16,5\times101\right)\)
\(a=17\times1666,5\)
\(a=28330,5\)