Since $\frac{1}{308888888858}$ and $\frac{87253465793496875642}{308888888858}$ have the same denominator, subtract them by subtracting their numerators.
$$\frac{1-87253465793496875642}{308888888858}$$
Subtract $87253465793496875642$ from $1$ to get $-87253465793496875641$.
