Bunuel

In ∆ ABC shown above, AB = BC and base AC is equal to the altitude of the triangle from point B. If the coordinates of points A and C are (2, 5) and (6, 5) respectively, which of the following could be the coordinates of point B?
(A) (2, 7)
(B) (2, 8)
(C) (2, 9)
(D) (4, 8)
(E) (4, 9)
Attachment:
2017-12-15_1258_001.png
Four answers can be rejected quickly: A, B, C, and D.
The prompt makes it easy to find B's y-coordinate. We are given that the altitude from point B = base AC. Add the length of AC to 5 to find B's y-coordinate.
Length of AC: (6 - 2) = 4
Points A and C (and AC) are at y=5. Add the 4
(5 + 4) = 9 = B's y-coordinate
Eliminate answers A, B, and D, whose y-coordinates are not 9
Also eliminate Answer C (2, 9), for a number of reasons.* If B is at (2,9), directly above A, we now have a right isosceles triangle where AB = AC and hypotenuse BC does NOT equal AB. BC must = AC. Reject C.
Answer E is correct
AB = BC: ∆ ABC is isosceles. Its altitude is a perpendicular bisector of the base AC.
The altitude from B hits the base at AC's midpoint (4,5). The x-coordinate for AC's midpoint is (2+6)/2 = 4. The x-coordinate of B must be 4.
B is at (4,9)
Answer E
*If that doesn't stand out, think of side lengths and angle measures. AB = BC, so
∠A = ∠C (angles opposite equal sides are equal).
Imagine B "moving" to directly above A: ∠A gets bigger, ∠C gets smaller. Not possible; they're equal. Side AB gets shorter, side BC gets longer. Also not possible: AB = BC