algebra online difficulties

[warrior-disable]

three questions i am unable to understand so far.
1.subtract (10a^2-4) from (a^2-7a-9)
2.simplify. answer w/ positive exponents.
-21a^-2bc^5
---------------
14a^-6bc^-2
after simplifying the answer(w/ positive exponents)
a. the numerator is
b. the denominator is:
please help and explain

[Wave-disabled]

>>1.subtract (10a²-4) from (a²-7a-9)

In this case, you need to combine like terms: 10a² and a² are the same term so you subtract them. The same goes to the other term

>>2.simplify. answer w/ positive exponents.
-21a^-2bc^5 -3a-bc^5
------------ = ------------
14a^-6bc^-2 2a-3b(1/c^2)

[Wave-disabled]

>>2.simplify. answer w/ positive exponents.

-21a^-2bc^5
------------
14a^-6bc^-2

-3a-bc^5
------------
2a-3b(1/c^2)