Hi I was wondering how do I Solve this question. I have to solve for $a$. I can solve for it when there's one summation but now there are three. My guess is factoring out the $A$. Divide $s$ by the $3$ summations $X$ $3$.
The constants are given:  

$$i = 0.03$$
$$s = 100000$$
$$n = 12$$
$$\sum\_{k=1}^{8}a\cdot(1+i)^{\frac{-k}{n}}+\sum\_{k=13}^{20}a\cdot(1+i)^{\frac{-k}{n}}+\sum\_{k=25}^{32}a\cdot(1+i)^{\frac{-k}{n}}=s$$

Would this be equivalent?
 $$ a = s / (\sum\_1^8\cdot(1+i)^{\frac{-k}{n}} + \sum\_{13}^{20}\cdot(1+i)^{\frac{-k}{n}} +$$
 $$\sum\_{25}^{32}\cdot(1+i)^{\frac{-k}{n}}) $$