If the temperature is the same in Farenheit and Celsius, the variable F is equal to the variable C in the equation. Thus, you'll just need to replace one or both variables in the original equation and solve for the single variable. I'll just use x as the variable here:
F = 1.8C + 32
x = 1.8x + 32
subtract x from both sides
0 = 0.8x + 32
subtract 32 from both sides
-32 = 0.8x
divide both sides by 0.8
-40 = x
So, since we know x = -40 degrees, we also know that F = -40° and C = -40° too.