Answer:
√97
Step-by-step explanation:
By definition, the number
is the ratio between the circumference and the diameter:

This implies that

And so your circumference is 
1/2 t-4(2+t)=20
1/2 t-8-4t=20
least common multiple=2
t-16-8t=40
-7t=40+16
-7t=56
t=56/-7=-8
Answer: t=-8
To check
-8/2 - 4(2-8)=-4-4(-6)=-4+24=20