If c is a scalar and u is a vector, then ||cu|| = |c| ||u||.
(|…| means absolute value, ||…|| means magnitude)
We have
||u|| = √(5² + (-12)²) = √169 = 13
and with c = -3, it follows that
||cu|| = |-3| × 13 = 3 × 13 = 39
