i don't understand this

Y=mx+b
b=y-intercept (what y is when x is 0)
b= -7
m=slope
m=change in y divided by change in x
m=(y2-y1)/(x2-x1)
m=(17-5)/(4-2)
m=12/2
m=6
Y=mx+b
Y=4x+(-7)
Y=4x-7