What is the slope of the line with equation y=12x

  When you have an equation of the form y = mx + b (which you do in this case), the slope is always equal to the coefficient of x, which is m or 12 in this case. Since there is no "b" in your equation, you could say that b=0, and the line is known to cross the y axis at zero. 

In case you are interested, if the equation said y = 12 x + 3 the slope of the line would still be 12 but the line would cross the y axis at 3. If the equation said y = 12x -4, the line would have a slope of 12 and would cross the y axis at -4.