The slope intercept form of an equation is given as
y = mx + b
where b is the y intercept and m is the slope.
So you already have
y = mx + 5
You can find the slope of the line by using two points and the slope formula. You have two points -- the x intercept (3,0) and the y intercept (0,5). So now you can find the slope of the line by using
m = y1-y2/x1-x2
m = (5-0)/(0-3)
That will give you your slope and then you have the equation.