book Maths Puzzle Numerical Ability

i dont know how to solve this problem...x-1=3
x+4=4

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my question on my algebra is (Write an equation to model the relashionship in each table). 1 Let f:X to Y and g:y to Z be mappings between non empty sets such that g o f:X to Z is injective.
(i)prove that f is necessarily injective.
(ii)Give an ezample in w