First off all angles within a triangle added to together add up to 180 degrees.
Also an isosceles triangle has two sides of equal length (as shown by the two lines drawn through the sides of the triangle)
Since you are given the angle B as 46 degrees that means that angle C is also 46 degrees (since they both share the same base and since both sides of the triangle are equal).
When added together you can subtract this from 180 degrees.
So...
180-92=88
88 is your answer.