book Maths Puzzle Numerical Ability

i dont get how you do this problem;5+(-2).

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book Other Question

solve by graphing.

#1

x-y=3
x+y=5

#2

x+2y=6
2x-3y=26

#3

x=2y+4
y=2x-8

#4

x=y-1
3y=-2x-x I need a theorem for congruence of matrix(a big one)