exam Maths Puzzle Numerical Ability

I also need help with theese three if you dont mind me asking for it.

4(3 − 2) = 4(3) − 4(2)

9 + 8 = 8 + 9

4 + (2 + 6) = (4 + 2) + 6

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exam Other Question

I need to find the altitude of the triangle

13 by 10 by 13

I have no idea how to do this.
can someone PLEASE help me solve this system:
{ x+ y-4z=6
{2x-5y+3z=2
{ x-6y+7z=-4
thank you so much!