The general form of slope-intercept form is y=mx+b
m stands for the slope
b stands for the y intercept
They are giving you m so you can replace m with -4/5 ⇒ y=-4/5x+b
Now you just need to find b. They also give you the point (0,-3). This means that when x is 0, y is equal to -3.
Lets go back to y=-4/5x+b and replace x with 0 and y with -3.
⇒ -3=(-4/5)(0) + b
⇒ -3= 0+b
⇒ -3=b
⇒ b=-3
Now that you have m and b, you can find your final answer in the form y=mx+b
⇒ y=-4/5x + (-3)
⇒ y= -4/5x - 3