Question

What is the y-coordinate of the y-intercept of the line given by the equation y = 5x + 21?

Answer 1

y intercept = (0,21)
it's 21.

Answer 2

y-intercept is a point where line intersect y-axis when x=0
so you can plug in 0 for x to find y-intercept or if given is slope intercept form equation \[y=mx+b\] where b represent y-intercept
whatever number is replaced by b would be y-intercept