The general equation in slope intercept form is given by:

where m is the slope and b is the y intercept.
Now we are given slope as 3 and y intercept as 4, plugging these values in the slope intercept form, we get

Now if we compare the two equations, we see that option (b) y=3x+4 matches our requirement.
So answer is the equation with slope 3 and y intercept 4 is y=3x+4.