\[4a - 4c + ac - a^{2} = 4 \cdot (a - c) - a \cdot\]
\[\cdot (a - c) = (a - c)(4 - a)\]
\[При\ a = 3,5;\ \ c = - 1,5:\]
\[\left( 3,5 - ( - 1,5) \right)(4 - 3,5) = 5 \cdot 0,5 = 2,5.\]