Finding Standing Waves by Adding Two Waves

[robert25pl]

I have two waves in a string. One of them is given below, where x is in cm and t is in seconds.

$$y=40cos\frac{\pi}{4}\((0.5x-4t-0.4)$$

I have to find second to produces standing waves when added to the first wave. I know how to add two waves were they are given. Can I get hint on this one. Thanks

 

[lightgrav]

to get a standing wave, you need 2 waves with
opposite direction of propagation; an ideal string
propagates all waves at same speed.
It is certainly easier to add waves of same frequency,
and even easier if their amplitude is the same.
You might want to adjust the phase (time offset)
of the second wave as they're added (to get sin/cos).

 

[robert25pl]

So all the values should be the same but with opposite direction. For phase should I used any number as long I get sin and cos after adding them up?