Another great answer Heureka :)
I have a couple of questions about your coding Heureka 
1) I have trouble remembering \cdot because it means nothing to me. Do you think the c stands for centre ?
Are there any other types of dots that you can have?
2) What is \text{$ for? What does it do, I see no text.
3) You have your alignment as rcl right, centre, left but there are no & symbols.
I experimented with leaving out the cl and it looked just the same - do they do anything?
Thank you :)
$$\\\small{\text{$\begin{array}{rcl}\dfrac{ 12\cdot a^3 \cdot b^2 }{ 4 \cdot a \cdot b^{-2} } \\\\
=\dfrac{ 12 } {4} \cdot \dfrac{ a^3 }{ a } \cdot \dfrac{ b^2 }{ b^{-2} } \\\\
=\dfrac{ 3\cdot 4 } {4} \cdot \dfrac{ a\cdot a^2 }{ a } \cdot \dfrac{ b^2 }{ b^{-2} } \\\\
=\dfrac{ 3\cdot \not{4} } {\not{4}} \cdot \dfrac{ \not{a}\cdot a^2 }{ \not{a} } \cdot \dfrac{ b^2 }{ b^{-2} } \\\\
= 3\cdot a^2 \cdot \dfrac{ b^2 }{ b^{-2} } \\\\
= 3\cdot a^2 \cdot b^{2-(-2) } \\\\
= 3\cdot a^2 \cdot b^{2+2} \\\\
= 3\cdot a^2 \cdot b^4 \end{array}$}}$$
\\\small{\text{$\begin{array}{rcl}\dfrac{ 12\cdot a^3 \cdot b^2 }{ 4 \cdot a \cdot b^{-2} } \\\\
=\dfrac{ 12 } {4} \cdot \dfrac{ a^3 }{ a } \cdot \dfrac{ b^2 }{ b^{-2} } \\\\
=\dfrac{ 3\cdot 4 } {4} \cdot \dfrac{ a\cdot a^2 }{ a } \cdot \dfrac{ b^2 }{ b^{-2} } \\\\
=\dfrac{ 3\cdot \not{4} } {\not{4}} \cdot \dfrac{ \not{a}\cdot a^2 }{ \not{a} } \cdot \dfrac{ b^2 }{ b^{-2} } \\\\
= 3\cdot a^2 \cdot \dfrac{ b^2 }{ b^{-2} } \\\\
= 3\cdot a^2 \cdot b^{2-(-2) } \\\\
= 3\cdot a^2 \cdot b^{2+2} \\\\
= 3\cdot a^2 \cdot b^4 \end{array}$}
+118724 
+118724 
