The number of cells of strain A follow the linear model \(C_A=9000-3000t\), where \(C_A\) denotes the number of cells of that strain and \(t\) is time in hours. Meanwhile, strain B follows \(C_B=4000-2000t\). The time \(t\) when both strains have the same number of cells is the intersection of these two lines, or the solution to
\(9000-3000t=4000-2000t\)
You have
\(5000=1000t\implies t=5\)
so after 5 hours, both strains should have the same number of cells.