Answer
12 mm
Work Step by Step
$h=b-2$
$a=b*h*1/2$
$60 = b*h*1/2$
$60*2=b*h*1/2*2$
$120 = b*h$
$120 = b*(b-2)$
$120 = b^2-2b$
$b^2-2b-120=0$
$b^2-12b+10b-120=0$
$b(b-12)+10(b-12)=0$
$(b-12)(b+10)=0$
$b-12=0$
$b=12$
$b+10=0$
$b=-10$ (invalid since we can't have a negative length)
