Hey Tro2,
Lets find the product of your equation together,
The question we are encountering is : What is the product (9t-4)(-9t-4)?
Lets simplify this step by step, shall we.
[tex]\left(9t-4\right)\left(-9t-4\right)[/tex] : Equation
[tex]\mathrm{Distribute\:parentheses\:using}:\quad \left(a+b\right)\left(c+d\right)=ac+ad+bc+bd[/tex].
Where : [tex]a=9t,\:b=-4,\:c=-9t,\:d=-4[/tex] .
This leads to [tex]9t\left(-9t\right)+9t\left(-4\right)-4\left(-9t\right)-4\left(-4\right)[/tex].
Now we have to [tex]\mathrm{Apply\:minus-plus\:rules}[/tex].
Where: [tex]-\left(-a\right)=a,\:\:\:\:+\left(-a\right)=-a[/tex]
[tex]-9t\cdot \:9t-9t\cdot \:4+4\cdot \:9t+4\cdot \:4[/tex]
[tex]=-81t^2+16[/tex]
