I have no idea where to even start. He gave us a 4 minute long video explaining absolutely nothing.

Answer:
x=3.6
Step-by-step explanation:
The triangles are similar meaning they are proportional, think like a ratio. So the one side is 1 and its proportional side is 20. That means you take a side of the small triangle and multiply by 20 to get the corresponding side on the large triangle. So, the small side x-3, multiply it by 20 to get the larger side.
20(x-3)=4x-2
20x-60=4x-2
16x=58
x=3.6