2x-12=34 is the equation.
x=23
Explanation: "Twelve decreased" means that the number 12 is taken away from a certain amount. This certain amount is "twice a number" which means double a value. This value is the number that we do not know, so I used 'x' as a variable placeholder for this number. This is how I got the equation.
I then solved the equation using algebra to find the value of x. You begin this process by adding 12 to 34, which equals 46. I then divided this number by 2, which equals 12. This is the most simplified the equation can get, so 12 is the value number we are looking for.