How to find x in the following

$_2F_1(b_1, b_2, b_3, b_4 - x)= a$

In other words, I need to find x that makes the above expression equivalent to a for a given $b_i$ for i=1, 2, 3, 4. I tried to use Solve function, but it didn't work!!

Thanks in advance

How to find bConst in the following code

For[QdB = -10, QdB <= 15, QdB++;
QConst = 10^(QdB/10);
Solve[1 - (bConst - 2/QConst )*Beta[1, 2]*
 Hypergeometric2F1[2, 1, 3, 1 - bConst] == 0.8, bConst];
Print[bConst]
]

I need to find bConst that makes the above expression equivalent to 0.8. I used Solve function, but it didn't work!!

Thanks in advance