[color=#0000ff]Example 1.[/color] [br][br] [math] 5a+8a-3a=(5+8-3)a=10a[/math][br] [br][color=#0000ff]E[color=#0000ff]xample[/color] 2.[/color][br][br] [math] \large 7bx+3ax^2-2bx+6ax^2=\underbrace{3ax^2+6ax^2}_\text{like terms}\underbrace{+7bx-2bx}_\text{like terms}=9ax^2+5bx[/math][br][br][color=#0000ff]E[color=#0000ff]xample[/color] 3.[/color][br][br]  [math]\large \begin{eqnarray}[br]&&3x^2-2x+6-(5x^2-6x-2)\\[br]&=&3x^2-2x+6\textcolor{red}{-}5x^2\textcolor{red}{+}6x\textcolor{red}{+}2\\[br]&=&\underbrace{3x^2-5x^2}\underbrace{-2x+6x}+6+2^\\[br]&=&-2x^2+4x+8[br]\end{eqnarray}[/math][br][br]Notice the minus-sign before the brackets![br][br][color=#0000ff]E[color=#0000ff]xample[/color] 4.[/color][br][br]  [math]\large\begin{eqnarray} 2a(3a^2-5a+7)&=&2a\cdot 3a^2+2a\cdot(-5a)+2a\cdot 7\\&=&6a^{1+2}-10a^{1+1}+14a\\&=&6a^3-10a^2+14a\end{eqnarray}[/math][br][br][color=#0000ff]E[color=#0000ff]xample[/color] 5.[/color][br][br] [math]\large\begin{eqnarray}[br]&&(2x-3)(4x^2+6x-9)\\[br]&=&2x\cdot 4x^2+2x\cdot 6x+2x\cdot (-9)-3\cdot 4x^2-3\cdot 6x-3\cdot (-9)\\[br]&=& 8x^3+12x^2-18x-12x^2-16x+27\\[br]&=&8x^3\underbrace{+12x^2-12x^2}\underbrace{-18x-18x}+27\\[br]&=&8x^3-36x+27[br]\end{eqnarray}[br][/math][br][br] [br]