Try either the Law of Sines or the Law if Cosines to use interior angles to create a ratio to the side proportions.
First, we convert the interest such that it is compounded annually. The formula would be:
ieff = (1 + i/m)^m - 1
where m = 4, since there are 4 quarters in a year
ieff = (1 + 0.025/4)^4 - 1
ieff = 0.0252
Then we use this for this equation:
F = P(1 + i)^n, where F is the future worth, P is the present worth and n is the number of years
F = $600(1 + 0.0252)^15
F = $871.53
Let

Ben's age
Let

Daniel's age
We know:

and

Plug the value of

into the second equation:





Since Ben is 20 years older than Daniel, Ben is 13+20=33 years old.
The cross of the position vectors is
.. [5, 2, 2] × [6, -1, 1] = [4, 7, -17] . . . . . the normal vector of the desired plane
Since the origin is a point in the plane, its equation can be written as
.. 4x +7y -17z = 0