I started learning OpenGL ES and Blender

Made a simple house in blender and able to render in OpenGL ES - Image below

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For now am just looking at front wall - Divided in 2 parts the bottom rectangle and the top Triangle

Now, i am stuck at this part : My objective is to increase height of top triangle, so to this i scale only the triangle part, but the thing is the top triangles alignment changes (Translate happens it seems) by scaling it...Like below image

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However the desired output should be - Image below

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Any ideas on what i am doing wrong ?

Thanks...