Question

How do I write an equation in slope intercept form from; Slope 3, y-intercept at -4

Answer 1

Slope-intercept form is y=mx+b
the y-intercept is b
the slope is m
enter these in, and you have y=3x-4

Answer 2

The slope-intercept form of the equation for a line is

y = (slope) x + (intercept).    <== THIS is the important thing to remember.

You already have both numbers to plug into that form.
The equation of your line is
                                                   y = 3x - 4

That's all there is to it.