Answer:
The age difference will be 11.35 years
Explanation:
From the point of view of Earth, Speedo's ship travelled 19.5 ly at 0.930c, thus it took it 19.5/0.930 = 20.97 years to arrive.
From the point of view of Earth, Goslo's ship travelled 19.5 ly at 0.715c, thus it took it 19.5/0.715 = 27.27 years to arrive.
But from Speedo's point of view it took him a different time to get to Planet X. This is calculated by the inverse of the time dilation transform:
[tex]T0 = T * \sqrt{1-\frac{v^2}{c^2}}[/tex]
[tex]T0 = 20.97 * \sqrt{1-\frac{0.93c^2}{c^2}} = 7.708 years[/tex]
And for Goslo:
[tex]T0 = 22.27 * \sqrt{1-\frac{0.715c^2}{c^2}} = 19.06 years[/tex]
The age difference will be 19.06 - 7.708 = 11.35 years