@Amit Mehta If you carefully notice, the first straight line $(L=1)$ divides the plane into $2$ regions. Again, the 2^{nd} line intersects $1$ line, creating $2$ regions. The 3^{rd} creates $3$ regions. This question basically asks the number of regions that lines divide a plane into. It is given by n

The proper derivation is provided here: Number of Regions N Lines Divide Plane (cut-the-knot.org)