After staring at this for a few seconds, I think the equation
[tex]-5\sqrt{10}\cdot\sqrt{15}=[1]\sqrt{[2]}[/tex]
is asking you to simplify the left hand side so that it has a form matching the right hand side, and [1] and [2] are placeholders. My guess is that they're supposed to be integers.
Notice that
[tex]\sqrt{10}=\sqrt{5\cdot2}=\sqrt5\cdot\sqrt2[/tex]
[tex]\sqrt{15}=\sqrt{5\cdot3}=\sqrt5\cdot\sqrt3[/tex]
[tex]\implies\sqrt{10}\cdot\sqrt{15}=(\sqrt5)^2\cdot\sqrt2\cdot\sqrt3=5\sqrt{2\cdot3}=5\sqrt6[/tex]
Then multiplying by -5, we have
[tex]-5\sqrt{10}\cdot\sqrt{15}=-25\sqrt6[/tex]
so I believe [1] = -25, and [2] = 6.
