Integration by sub

[sweet garden]

Can someone explain this pls

[true crater]

Which part

[sweet garden]

Part a

[sonic ibex]

just do what you normally do for substitution
x^1/2 = a^1/2 * sin(theta)
dx/dtheta = 2asin(theta)cos(theta)
a-x = a(1-sin^2(theta) = acos^2(theta)
just simplify everything