300+5x-0.5x^2-30+0.5x=275
x^2- 1 1x+ 10=0
(x- 1)(x- 10)=0
X= 1 or x= 10.
That is, when the annual rent of each store is set at 6.5438+0.05 million yuan or 6.5438+0.5 million yuan, the annual income is 2.75 million yuan.