Hi,

to Q1:

the brackets are not necessary. Collect like terms:

7t^(-6)*(-3)t^(-3)*c^(-3) = -21t^(-9)*c^(-3)

to Q3.:

11x + 6y = 153

156 = -3y + 10x ==> 156 - 10x = -3y ==> -312 + 20x = 6y

Now plug in the term for 6y into the first equation:

11x + (-312 + 20x) = 153

31x = 465

x = 15 plug in this value into the second equation to calculate y:

6y = -312 + 20*15 = -12

y = -2

EB