Use the law of universal gravitation, which says the force due to gravity between two bodies of masses m₁ and m₂ separated by a distance r is
F = G m₁ m₂ / r²
(where G = 6.67 x 10⁻¹¹ N m²/kg²)
as well as Newton's second law, which says a force that acts on a body of mass m is proportional to its acceleration a such that
F = m a
So we have
G m₁ m₂ / r² = m a
m is the same mass as one of the masses used on the left side:
G m / r² = a
a = (6.67 x 10⁻¹¹ N m²/kg²) (1.024 x 10²⁶ kg) / (2.5 x 10⁷ m)²
a ≈ 10.93 m/s²