So I was going through questions in a GRE prep book and I'm just convinced this answer is wrong but I understand the likelihood is very slim. Can someone check my work and explain why I am wrong? 
So I am given this isosceles triangle and I am told the measure of the angles are expressed by 2x+y=180. Since the triangle is isosceles I know the perimeter would either be represented by 7+7+4=18 or 7+4+4=15. Now from here can't I drop an altitude from R and figure out which perimeter correctly represents the triangle?
I showed my work however the answer key says the relationship between Quantity A and Quantity B cannot be determined.
The problem is that the picture implies that $7$ is the duplicated value, but nothing in the body of the question implies that. So yes, the triangle could be $4,4,7$ or $4,7,7$, so it could have perimeter $15$ or $18$.
Basically, this is a trick question, reminding you that you should not draw any inference from the picture of the problem, except the specific lengths (4,7) listed and the fact given in the text.
The picture is not of a $4,4,7$ triangle, because it is not obtuse, but the picture is not something from which you should use when making geometric inferences.