GRE Sample Test 2 Quantitative Reasoning 2

Because you know the perimeter of the rectangle, you can figure out that both BC and AD = 5. Thus, the area of the rectangle is 3 × 5 = 15. The area of the triangle is therefore also 15. Because the area of a triangle = $\frac{1}{2}$ bh, you can put in the values you know to find 15 = $\frac{1}{2}$ (b × h) and solve for the base, which is 6. LM is the base of the triangle, so LM = 6.