[tex]3 x^{5} +6 x^{2} -5-[2 x^{4} +7 x^{2} -x+16][/tex]
I put the second set in brackets because you have to distribute the negative:
[tex]3 x^{5} +6 x^{2} -5-2 x^{4} -7 x^{2} +x-16[/tex]
Now combine like terms and put it in the right order:
[tex]3 x^{5} -2 x^{4} - x^{2} +x-21[/tex]
