Answer
(a) 2000.
(b) 2008.
Work Step by Step
(a) Given $R=0.2844x+5.535$, to exceed $7.6\ billion$, let $R\ge7.6$, we have $0.2844x+5.535\ge7.6$ which gives $x\ge7.26$. Thus it needs at least 7 years or until 2000.
(b) let $R\ge10$, we have $0.2844x+5.535\ge10$ which gives $x\ge15.7$. Thus it needs at least 15 years or until 2008.