book Maths Puzzle Numerical Ability

construct a 3x3 non zero matrix A such that the vector [1 -2 1] (vertical column not horizontal) is a solution of Ax=0

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book Other Question

2 t
--- = ----
t 5t-12

on this one you take 2 divided by t = t divided by 5t-12
I just need to know what to do with this problem. Thanks.
Find the perimeter of the rectangle if a = 4 and b = 7?