First, I would find the measure of the third angle, as it is opposite the side you already know.
180= 90 + 28 + x
90-28=x
x=62 degrees
Now, we use the law of sines to find the length of the missing side.
x/sine(90)=11/sine(62)
x=11(sin(90))/sin(62)
x=11(.5)/sin(62)
x=5.5/0.883
x=4.85
The length of the missing side is 4.85