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.