The equations fro each tree will be the inital hight plus the hight they grow so:
[tex]\begin{gathered} A=24+6t \\ B=39+3t \end{gathered}[/tex]Now we need the time when A is bigger than B, so first we are going to find the time when they are equal so A=B
[tex]\begin{gathered} 24+6t=39+3t \\ 6t-3t=39-24 \\ 3t=15 \\ t=\frac{15}{3}=5 \end{gathered}[/tex]So in 5 years they are going to have the same height, so the solution will be 6 years when A is taller than B
[tex]t>5\text{ years}[/tex]