Let c= speed of current and v=speed of canoe, their speed is v+c downstream and v-c upstream.
33=6(v+c) and 20=8(v-c).
33=6v+6c and 20=8v-8c.
So 11=2v+2c and 5=2v-2c; adding these: 16=4v so v=4mph.
And c=½(8-5)=1.5mph.
Canoe speed=4mph, speed of the current=1.5mph.