I'm assuming you mean simplify? I'm gonna do that.
\(-6t*4m*(-m)\)
-m can be expressed as -1*m, and -6t can be expressed as -1*6t: \(-1*-1*6t*4m*m\)
-1 * -1 = 1: \(1*6t*4m*m=6t*4m*m\)
m * m = m2: \(6t*4m*m=6t*4*(m*m)=6t*4*m^2\)
Factor out 2: \(2(3t*2m^2)\)
The simplest form of the expression is \((2(3t*2m^2))...or...(6t*4m^2)\)