Those illustrations in video 30 are to compare the Vedic and conventional methods. In fact the solution of quadratics shown in this lesson is for when the coefficient of x^2 is 1, and in the illustration it is 3.
We cover this fully in the Advanced course but since you ask let me explain.
The next step is -50 divided by 13, which gives -4 remainder 2.
The next step differs in that when we subtract the Duplex of -4 from 20 we have to treble it first so we have 20-48=-28 and -28 divided by 13 gives -3 remainder 11.
Since you may ask why we say -28 divided by 13 gives -3 remainder 11 rather than -2 rem -2, this becomes clear once you start to proceed to the following step.