e^(10)-e^(10-x)-x/(e^(10)-11)=0.25

and
e^(10)-e^(10-x)-x/(e^(10)-11)=0.75
-Solve both for x.
-Can you solve both of these for x without a Ti-89 calculator? If so how?
-The reason I want to calculate this is to find upper and lower quartiles of a deformity angle which is defined by the function on the left hand side of the equals sign.

You can certainly solve for x without a TI-89, but you will need to use some kind of iterative method.

Judging by the shape of the curve, Newton's method should converge quite rapidly.