9=12.57/(1+4.11e^-0.026x) solve for x

Solve for x:
9 = 12.57/(4.00452 x+1)

9 = 12.57/(4.00452 x+1) is equivalent to 12.57/(4.00452 x+1) = 9:
12.57/(4.00452 x+1) = 9

Take the reciprocal of both sides:
0.0795545 (4.00452 x+1) = 1/9

Divide both sides by 0.0795545:
4.00452 x+1 = 1.39667

Subtract 1 from both sides:
4.00452 x = 0.396667

Divide both sides by 4.00452:
Answer: |x = 0.0990548

If you meant to write your equation this way, then you get:

9=12.57/(1+4.11e^(-0.026x)), solve for x, then:

x== 89.92.....etc.