To construct a triangle when its perimeter and two base angles are given, the first step is to draw a segment equal to the ____.

When the perimeter and two base angles are given, first draw a segment XY equal to the perimeter. Construct the two given base angles at X and Y and bisect them; the bisectors meet at the vertex A. The perpendicular bisectors of AX and AY then cut XY at the other two vertices B and C.