Hello,

A product is negative if the two factors are of different sign.

-> T-7>0 and T+1<0

Or T-7<0 and T+1>0

For this one,x^3 + 36 is greater than to 4x^2 + 9x

x^3+36>4x^2+9x

<=> (x^3-9x)+(36-4x^2)>0

<=> x(x²-9)+4(9-x²)>0

<=> (x²-9)(x-4)>0

A product is positive if its factors are of the same sign.

-> x²-9>0 and x-4>0

Or x²-9<0 and x-4<0