On a spacecraft two engines fire for a time of 541 s. One gives the craft an acceleration in the x direction of ax = 5.13 m/s2, while the other produces an acceleration in the y direction of ay = 7.28 m/s2. At the end of the firing period, the craft has velocity components of vx = 3716 m/s and vy = 4869 m/s. Calculate the magnitude of the initial velocity.