The area of a trapezoid is: (base1+base2)/2*height=area
So, let's plug in the values and put in the unknown value as a variable :)
(x=the unknown base length)
(10+x)/2 * 4=48
20+2x=48
2x=28
x=14
So, the other base would have a length of 14 units.
Please correct me if I'm wrong :)