Second case:
Distance=600miles,Speed=(x+20)mph
Time=(600/x+20)hours which we know is 1 hour less than the first case
So we get,
[This is your required equation]
This is a quadratic in x and can thus be solved as:
So we get x=100 or -120 mph
This speed cannot be negative, thus x=100
Hope this helps,
prabhat