exam Maths Puzzle Numerical Ability

How do I answer this problem.
y^4+3y^3-5y^2-15y

I tried to solve it this way but I think that it is wrong.

y^4 + 3y^3 -5y^2-15y
y^3(y+3) -5y(y+3)

=(y+3)(y^3-5y)

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exam Other Question

an airplane manufacture sells small planes that have seats arranged so there is a center aisle. in the each row there are 3 seats on the left side of the aisle and 2 seats on the r using cramers rule, solve

2x - y = 4
3x + y - z = 10
y + z = 3