Trapezoid ABCD has vertices A(-1,0), B(0,4), C(m,6) and D(k,0), with m>0 and k>0. The line y = -x + 4 is perpendicular to the line containing side CD, and the area of trapezoid ABCD is 38 square units. What is the value of k?
Hint: The slope of line CD is 1. Then, you can set up two equations using the area and the slope of line CD.