We can represent complex numbers geometrically by plotting them on the "complex plane", just like we plot points on the Cartesian plane. The real part of the complex number is the horizontal coordinate and the imaginary part is the vertical coordinate. So, the complex number 0 is the origin. The number 2-3i is plotted below.
We say that the magnitude of a complex number is the distance from that complex number to the origin in the complex plane. We denote the magnitude of the complex number a+bi as the absolute value of a+bi.
Compute $|5-12i| + |3-4i|$