Past exam of the mathematics course of the University of Cambridge / 2015 / ii / Paper 1 / 14I / a / Solution

A strongly regular graph with parameters $(k,a,b)$ is a simple graph in which each vertex has degree $k$, adjacent vertices have exactly $a$ common neighbours, and distinct nonadjacent vertices have exactly $b$ common neighbours. Its order $n$ is often included separately as a fourth parameter.