The ratio of boys to girls is 12 to 16, 12:16 or 12/16 (all different ways this can be written). The fraction form 12/16 can be reduced by dividing by the greatest common factor. Both 12 and 16 are divisible by 4, so the reduced fraction is 12/4 over 16/4 = 3/4. In other words, there are 3 boys for every 4 girls in the class.
The equation is already is slope intercept form where you can easily point out the gradient, or the slope, and the y intercept. y=mx+b where m is the gradient, and b is the y intercept. Since there is no m, the gradient is 1. The y intercept had x=0 and y=b b in this case is 6 the y intercept= (0,6)