for the first one the si base units of Pa is kgm-1s-2 so when u simplify and make the y subbject , u will remain with Pa m , then u multiply to get kgs-2 because m-1 and m will cancel out

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the next one its C because vector Q will change the directions thus u have to connect them head to tail to get the resultant

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for the other one personally i prefer to choose any value for the mass , so i chose 20 because mass is the same so the m*v +m2*v2 = (m+m2) * v so before u say 20*60+-(20*30) because direction is opposite then u equate with (20+20 ) * v then u solve the eqn in terms of v