How do I get P from distinct EV

[SwissPoker]

I need a formula to determine P in case I have a,b, and c:

a = -20
b = +80
c = +54

EV = (P)(a)+(1-P)(b) = c

How do I determine P? What is the formula?
Thanks for any help.

P = (c - b) / (a - b)
or 0.26