Projectile Motion in Relativity: Lorentz Vector Subtraction

[ZedCar]

Homework Statement

A projectile is fired from a canon at a velocity of 0.4c at an elevation angle of 30° to the X-axis. An observor is moving at a velocity of 0.1c relative to the canon along the X direction.
(a) At what angle does the observor measure the projectile to be fired?
(b) What is the projectile’s speed relative to the observor?

Homework Equations

The Attempt at a Solution

Am I correct in thinking that I use the same method of Lorentz vector subtraction as shown near the bottom of the linked-to webpage below?

http://www.relativitycalculator.com/addition_relativistic_velocities.shtml

i.e. the section entitled, "What, therefore, would r & θ appear to be to an observor in relatively moving S'?"

Example 1

Or does anyone suggest another (simpler?) method?

Thank you.

 

[ZedCar]

Sorry, hadn't added in the linked-to webpage, which I've just done!