Answer:
It is given that the volume of a cone =
cubic units
Volume of cone with radius 'r' and height 'h' = 
Equating the given volumes, we get

× 

It is given that the height is 'x' units.
Therefore, 

Therefore, 
So, the expression '
' represents the radius of the cone's base in units.
7.87 is what i got :) good luck!!
He just switched sides of the variables and numbers. Instead if adding the opposite.
12 + 0.35X = 20.05
12 + 0.35X - 12 = 20.05 - 12
0.35X = 8.05
0.35X/0.35 = 8.05/0.35
X = 23
Hope this helps!!