Question

find BC,if the area of the triangle ABC is cmsquare and hight AD is 6 cm

Answer 1

but what is the area of the triangle i think so you missed to write the area of triangle.
AD is the perpendicular to BC
area of triangle ABC = 1/2  *  AD*BC
                                 = 6*BC/2
                                 = 3BC
  when you know the area of triangle ,
area of triangle/3  gives you the side BC.

Answer 2

See...

Area of triangle is ½*base*height
Here, AD is the height that is 6cm
Since you missed the area in the question by mistake (which is okay)... let the area be x.
Base is BC which is to be found.

So according to the formula,
½* BC * AD=x
BC*AD= 2x
6BC=2x
BC=2x/6
BC=x/3 cm

Now you tally your information, and put the value of area where x is and solve further, or provide the area in the comments box of this question, I'll edit the answer accordingly.

Hope it helped! Mark the Brainliest if it did. Good luck! :)