First find the value of x. Expanding and combining like terms, we find that it becomes a quadratic equation, 5x2+6x−18=0.
Solving using the quadratic formula, we see that x has two solutions- −3±3√115. We want the largest possible solution, so the ± becomes a + instead.
Thus we have −3+3√115=a+b√cd. Substituting these values in for each other, we find that a=−3,b=3,c=11,d=5. So the value of acdb=−3⋅11⋅53=−55.
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