@acer How can I joint two commands  Display(temp, inert=false) and value(temp); to get answers like this x^2 - 2x - 3x + 6 = x^2 - 5 x + 6.

Consider f(x) = (x-a)^4  + (x-b)^4 - c;

We have

f'(x) = 4*(x - a)^3 + 4*(x - b)^3;

f''(x) = 12*(x - a)^2 + 12*(x - b)^2 >=0
This means that the graph is concave up everywhere, therefore f(x) = 0  cannot have more than two real roots.

@Carl Love 

@Carl Love I try with your c. I got