Maleeha,
x(x5)>36 if
x^{2}  5x  36 > 0 which is
(x  9)(x + 4) > 0
The product of two numbers, here (x  9) and (x + 4), is positive if either both numbers are positive or both numbers are negative. Hence you have two cases to consider:
Case 1: (x  9) > 0 and (x + 4) > 0
(x  9) > 0 means x > 9 and x + 4 > 0 means x > 4.
Hence (x  9) > 0 and (x + 4) > 0 if x > 9 and x > 4.
But this is just x > 9.
Case 2: (x  9) < 0 and x + 4 < 0
This is yours to complete.
Now can you complete part b?
Penny
