Question 182693
 please help me solve these problems 
1. (2-3)-(2x-9) 


2. (a-3)(a+4)(a-5)
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What do you want to do?  There's nothing to solve for, they're not equations.
I'll expand #2
(a-3)*(a+4)*(a-5)
= {{{(a-3)*(a^2 - a - 20)}}} You can multiply them in any order.
={{{a^3 - 4a^2 -17a + 60}}}