Factorise: a squared + 8a - 20

Factor out the 4 in both equations

8a^2-20^2 = (2^2 times a^2 times 2) - (2^2 times 5)

therefor it is also equal to

(2a) ^2 times 2 - (2^2 times 5)

we can force it into a difference of 2 perfect squares which is a^2-b^2 = (a-b) (a+b)

(2a√2) ^2 - (2√5) ^2 = ((2a√2) - (2√5)) ((2a√2) + (2√5))