Question 196742
a) {{{N=  x^(1/3) + 2x + 2}}}
{{{N = 16^(1/3) + 16*2 + 2}}}
{{{N = 2.52 + 32 + 2}}}
{{{N = 36.52}}}million

b) You could solve this algebraically and the plot it OR you could think a little and find the answer emperically (trial and error). In this case, trial and error is quicker.
Try 1. Then N = 1+2+2 = 5 (not quite 7)
Try 2. Then N = 1.25+4+2 = 7.25
So the answer is 'a little less than 2 years'. 
Try 1.9. Then N =   1.23 + 3.8 + 2 = 7.03
Close enough for me. 1.9 years works :)