We can use the equation:

Age is given as 800
Circumference is 2*pi*r
Growth rate is the rate of change of growth of the tree. We can find this by looking at the linear equation.
The equation is:
y = 4.39x - 6.77
Matching with y = mx + b, we can say:
m = 4.39 [m is the rate of change. So, this is the growth rate]
Now, we substitute into equation to solve for r:

Diameter is double this radius. So,
Diameter = 1249.74 * 2 = 2499.48
Diameter = 2499.48