Solve for x: 54=(x+10)(2x-3)

54=x*2x+10*2x-3*x-3*10
54=2x^2+20x-3x-30
54=2x^2+17x-30
2x^2+17x-30-54=0
2x^2+17x-84=0
x^2+8.5x-42=0

to see how to solve quadratic equations see:  http://en.wikipedia.org/wiki/Quadratic_equation

formula:
x^2+p*x+q=0
x₁=-p/2+sqrt( (p/2)^2-q )
x₂=-p/2-sqrt( (p/2)^2-q )

with p=8.5 and q=-42
x₁=-12
x₂=3.5

$${\mathtt{54}} = \left(\left({\mathtt{x}}{\mathtt{\,\small\textbf+\,}}{\mathtt{10}}\right){\mathtt{\,\times\,}}\left({\mathtt{2}}{\mathtt{\,\times\,}}{\mathtt{x}}{\mathtt{\,-\,}}{\mathtt{3}}\right)\right) = \left\{ \begin{array}{l}{\mathtt{x}} = -{\mathtt{12}}\\ {\mathtt{x}} = {\frac{{\mathtt{7}}}{{\mathtt{2}}}}\\ \end{array} \right\}$$