exam Maths Puzzle Numerical Ability

i was having trouble with this problem(Factor Completely)

5a^2-2ab-7b^2

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exam Other Question

Could I please have some help with the following problem.
Let P={-3,-2,0,1,5} Let T={0,1,6,9} Let S={-3,1,6}
Find T and S
Find P and T 1)Given the binary operation * defined over the set of real numbers R by a*b=a+b+ab. What are the invertible elements of R with respect to *.