how to find y intercept of line y= 4x+12

Answer 1

For any equation in x and y,
to find the x-intercept, let y = 0 in the equation
to find the y-intercept , let x = 0 in your equation

of course you should be familiar with the form y = mx + b
where b is the y-intercept
For yours it would be 12

Answer 2

y = mx+b
is called the slope-intercept form for a reason ...