1 equation 1 unknown but still killing my brain

hi all. i have the following equation and am not sure how to solve it. it seems so simple but i haven't done math in a while and am not sure the steps to solve this. I can create a number close enough using excel to solve it through trial and error which is enough to tell me that it is possible, but now i just want to know the steps/process:

100+100x+100x^2+100x^3+100x^4 = 400

Essentially I have the first month's 'investment', and the total at the end of 5 months. Since I know that each subsequent month's investment is smaller than the first, I am trying to solve for the linear rate of decline...please help, and let me know if you have questions on what i'm trying to achieve. The numbers above (100 and 400) are arbitrary and I just picked them entirely at random for the purpose of simplification.