exam Maths Puzzle Numerical Ability

i do not understand the (3) substitional methods.i really do need help asap...

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exam Other Question

1.Find the value of 20x when x=10?

2.find the value of 3a when a=-4?

3.find the value of 11(a+b) when a=7 and b=3?
pentagon, hexagon, octagon constructions with compass and steps of constructions