a, <=> 4x = 8 <=> x = 2
b, => 10x+15=20+4x-28 <=> 6x = -23 <=> x = -23/6
c, \(\Leftrightarrow\left\{{}\begin{matrix}3x-36=0\\1-2x=0\\x+1=0\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}x=12\\x=\dfrac{1}{2}\\x=-1\end{matrix}\right.\)
d, <=> (x-1)(x-4) <=> x = 1 ; x = 4