When you have surf, go back to Hayward city and surf to the house you couldn't get to before. Inside there's a man, who will give you strength, if you return his thing (i'm sorry, i can't remember what it's called - it's something with letters and numbers). It is found in the safari zone, and you have to have go-goggles to find it.
In the safari zone, you go through the cave and on the other side, there is a sandstorm. It's in that area, the mans thing is. When you got it, you return it to him and he will give you strength.
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It is found in the ice-part if rijon tunnel. You have to have both strength and rock smash to get it.
HM STRENGTH HM STRENGTH
you get hm strength and hm defog after defeating two gym leader.Fantina(defog) Byron(strength).
get the HM strength, and teach it to a Pokemon
You get the hm strength and give it to one of your pokemon