Answer:
The fifth line contains the error.
Step-by-step explanation:
Line 1 is the equation.
Lines 2, 3, and 4 are correct.
The error is in the 5th line.
The square root of a negative number is not a real number, so if this problem is solved only with real numbers, there is no solution.
If imaginary numbers are allowed, then the last line should read:

Answer: option d. the argument is valid by the law of detachment.
The law of detachment consists in make a conlcusion in this way:
Premise 1) a => b
Premise 2) a is true
Conclusion: Then, b is true
Note: the order of the premises 1 and 2 does not modifiy the argument.
IN this case:
Premise 1) angle > 90 => obtuse
Premise 2) angle = 102 [i.e. it is true that angle > 90]]
Conclusion: it is true that angle is obtuse
The answer for your question is going to be. 2^((3)/(2)) I know this cause I just took a quiz with this question on it
Statement 3: tri egc = tri egb
statement 4: tri ebc is equilateral.
reason 5: equilateral triangles have equal angles