How do i write an equation of a line that goes through point (4,-5) and is parallel to y=2x+8?

So you know the new equation must have the same slope as the old equation since they're parallel to one another. Next just simply take the slope and the point given and plug them into the point-slope formula and you have your new equation:
y-y1=m(x-x1) --- Point-Slope Formula
y+5=2(x-4)
y+5=2x-8
y=2x-13 --- Answer

The answer will be y=-5+0 because the slope is -5 and the it has to go through y-axis 0