the answer is 8
we know that (a . b)^2 is equal to
a^2 + 2ab + b^2
[tex](2a + 3b) {}^{2} = 20 \\ 4a {}^{2} + 12ab + 9b {}^{2} = 20 \\ (ab = 1) \: \: \: = = > 4a {}^{2} + 9b {}^{2} = 8[/tex]
good luck
