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Forth one for the first one and third one for the second

(2x-3y) (4x-y)
Multiply the polynomials using distributive property.
(a+b)(c+d) = ac+ad + bc + bd
(2x-3y) (4x-y) = (2x) (4x) + (2x)(-y) + (-3y)(4x) + (-3y)(-y)
Apply plus/minus rule.
-(-a)=a and -(a)=-a
we get
=8x²-2xy-12xy+3y²
Add like terms
=8x²-14xy+3y²

8x² - 14xy + 3y²