When taking square roots, you can't take square roots of negative roots of negative numbers. So, what will work for the domain of u(x) is what makes u(x) zero or more. We can make an inequality for that.
u(x) ≥ 0.

9x + 27 ≥ 0 by squaring both sides
9x ≥ -27
x ≥ -3
So the domain of the function is when x ≥ -3 is true.
It could be 2+1 beacuse it says there are twice as many
1/3(21 - 3a) = 6 - a
7 - a = 6 - a
7 = 6
Therefore, the equation has no solution.
treat the tree trunk as a cylinder
v= pi x r^2 x h
using 3.14 for pi
3.14 x 3^2 x 10 = 282.6 cubic feet
282.6 * 45 = 12, 717 pounds