This involvea factoring the polynomial into binomials and solving.
So make it (3x+1)(x-7)
X equals 7 and -1/3
1500.56 x 12 = 18006.72 per year
18006.72 x 30 = 540201.60
540201.60-350000 = 190201.60
Recall the secant-tangent theorem, and you have
EA^2 = EC*CD
12^2 = 8*(x+10)
and now ED = EC+CD = 8+x+10
I suspect a typo somewhere in the murk above
