I'd really like some help in solving an integral.

ʃ 1/ √x (1-3√x)

In numerator: 1
In denominator: the square root of x times 1 minus 3 times the square root of x

The answer given is -2/3 ln l1-3√xl + C but I don't know how to get there.

u=1-3√x

du = -3/(2√x) = (-3/2)(1/√x)
you already have 1/√x, so the new integrand becomes

ʃ 1/u (-2/3)du

that should make it clear