I need to calculate the following expression. Is there any explanation to convert this expression into normal expression without those letters for sum and the product? Just normal expression.
$$ Z = \displaystyle\sum_{i=1}^{n} \displaystyle\prod_{k=1}^{i+2} (3k+2). $$
As I said in my comment, this is a "normal expression". This notation comes up a lot in any type of math and so you should get used to it. The summation notation sums the enclosed expression from the lower bound to the upper bound. The product notation does the same but for products. So, for example, $$ \displaystyle\prod_{k=1}^{i+2} (3k+2) $$ multiplies $3k+2$ for $k=1$, $k=2$, etc., but to $i+2$. This yields an expression in terms of the upper bound $i$, which then gets summed by the $ \displaystyle\sum $.