A right pyramid has a square base length of 9cm. The sloping edges are 15cm in length. Find the angle that the sloping edge makes with the b

wouldn't it be 65°??

The pyramid is square based. Therefore the diagonal between corners would be 12.73, because:

9² + 9² = 162

√162 = 12.73

So the adjacent side of the triangle would be 6.365 (half of 12.73) and the hypotenuse would be 15. 

$$\underset{\,\,\,\,^{\textcolor[rgb]{0.66,0.66,0.66}{360^\circ}}}{{cos}}^{\!\!\mathtt{-1}}{\left({\frac{{\mathtt{6.365}}}{{\mathtt{15}}}}\right)}$$  = 65 (to the nearest degree)

Is this right ^^ or am I doing something wrong?

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Yes anon I believe you are correct.    

It is a rare day that Heureka makes any error  - but it is very easily done