To find the the endpoint of a vector, you would project the endpoint to the x & y axes using sin & cos.

The x component is given by the magnitude of the vector times the Cos of the angle [8*Cos(45 degrees) in your example].

The y component is given by the magnitude of the vector times the Sin of the angle [8*Sin(45 degrees) in your example].

If the angle is always 45 degrees, you know that the x & y components will be equal and the equation simplifies to x2 = y2 = 8*0.7071 = 5.657 [because Sin(45 degrees) = Cos(45 degrees) = 0.7071].

Remember though: traditionally when working with vectors, positive y values are upwards, but with controls, positive y values are downwards.