exam Maths Puzzle Numerical Ability

ok so i have a factoring polynomials test and im so confued!! the problem is
9b^2-20 i know that its prime but i dont know why its prime! please help!!

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exam Other Question

write an equation in slope intercept form for the line that satisfies each set of conditions.

passes through (4,6) parallel to the graph of Y=2/3x+5
Use the factor theorem to decide whether or not the second polynomial is a factor of the first.
9) 4x^2 - 33x + 65; x - 5