Iforgot how to multiply fractions cause its been so one ! 3/4÷4/5.

Well, you're asking for a refresher on multiplying fractions, but then the
example at the end of your question uses the symbol for division, not
multiplication.  So I'll just give you the rules for both operations, and let you
choose the one you need.

To multiply fractions:

-- Multiply the two numerators. 
   Write the product on top of a new fraction.

-- Multiply the two denominators. 
   Write the product on the bottom of the new fraction.

-- The new fraction is the product of the two original fractions.

To divide fractions:

-- Invert (flip) the second fraction.

-- Then multiply them.

-- Their product is actually the quotient of the two original fractions.