we know that
If the microscope magnify the spider by [tex]4*10^{2}[/tex]
that means that magnifies the spider 400 times
so
Multiply the length of the tarantula by [tex]4*10^{2}[/tex]
[tex]30mm*4*10^{2} =30mm*400=12,000 mm[/tex]
therefore
the answer is
the spider under such microscope he will appear to be 12000 mm