What's the question? the estimated product of two numbers that are not multiples of 10 is 2800.

Seems to me like the question is:  What are the two numbers ?

They have to multiply to  2,800 . 
It's fairly easy to find two numbers that do that,  BUT ...
they're not supposed to be multiples of 10 .

1 x 2,800 = 2,800 . . . . . No good.  2,800 is a multiple of 10 .

2 x 1,400 = 2,800 . . . . . No good.  1,400 is a multiple of 10 .

4 x 700 = 2,800 . . . . . No good.  700 is a multiple of 10 .

5 x 560 = 2,800 . . . . . No good.  560 is a multiple of 10 .

7 x 400 = 2,800 . . . . . No good.  400 is a multiple of 10 .

8 x 350 = 2,800 . . . . . No good.  350 is a multiple of 10 .

10 x 280 = 2,800 . . . . . No good.  280 is a multiple of 10 .

14 x 200 = 2,800 . . . . . No good.  200 is a multiple of 10 .

16 x 175 = 2,800 . . . . . Finally !  Neither one is a multiple of 10 .

If you need an equation, it would be y=2x+8, y would be how long it grew, x being weeks.if you need how long he was asleep, it would be 4 weeks if he started with no beard, the equation would be x=8÷2if he did have a beard before hand, the equation would be 2w+x=8