SOLUTION: Since 10yr A will be twice as B.10yr ago if A is 9yr present age B.

I think it's supposed to be an equilateral triangle.

On my screen, which is a little larger than most,
I measured them and got:

AB = 3 7/8 inches
AC = 3 9/16 inches
BC = 3 1/2 inches

AB is longer, but maybe my screen stretches
horizontally.  I'd call it equilateral.

Edwin