Trigonometry

[fair stirrup]

use b^2 - a^2 = (a+b)(b-a)

[grand creek]

Equate the LHS with the given info in question

Then expand √(a²-b²) as √(a+b) (a-b)

And then cancel stuff

It should be simpler now

[zinc geyser]

@fair stirrup and @grand creek thankyou for helping:)