I need to find the equation of the line parallel to y=3 and it goes through points (2,-1) and put it in slope intercept form

Are you sure you have understood the problem correctly? The line y=3 is a horizontal line, and a horizontal line has a slope of zero. The only lines parallel to a horizontal line are other horizontal lines. Therefore, there is no slope. The slope is the coefficient of x in the slope-intercept form: y = mx + b. m stands for the slope, and b is the y-intercept.