exam Maths Puzzle Numerical Ability

Can someone explain the basic principles for the substituion method? I would like to know what can make it easier to understand if at all possible. Thanks!

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exam Other Question

find the slope of the line that passes through each pair of points.
(-2,-3),(0,-5)
A system of linear equations has solutions (1, -1) and (-2,3).

a)Can you find another solution?
b)How many solutions must exist?