Final answer:
The magnitude of -3i is 3.
Step-by-step explanation:
The magnitude of a vector is the length or size of the vector. To find the magnitude of -3i, we use the formula √(a² + b²), where a is the x-component and b is the y-component of the vector. In this case, the x-component is -3 and the y-component is 0. So the magnitude of -3i is √((-3)² + 0²) = √9 = 3.