The points X, Y and Z are located such that Y is 15km south of X, Z is 20km from X on a bearing of 270°. Calculate, correct

Let point X be the origin $(0,0)$.
Point Y is 15 km south of X. This means Y is at coordinates $(0, -15)$.
Point Z is 20 km from X on a bearing of $270^\circ$. A bearing of $270^\circ$ is due west. This means Z is at coordinates $(-20, 0)$.

Since Y is directly south of X and Z is directly west of X, the lines XY and XZ are perpendicular. Therefore, triangle XYZ is a right-angled triangle with the right angle at X.
The length of side XY is 15 km.
The length of side XZ is 20 km.

In a right-angled triangle, the square of the hypotenuse (YZ) is equal to the sum of the squares of the other two sides (XY and XZ).

The distance YZ is $25\, \text{km}$.