The p's in the denominator cancel and you're left with g alone.

\(\displaystyle H=\frac{1.85Q^{\frac{1}{4}}\cdot u^{\frac{3}{4}}}{g^{\frac{1}{2}}}\)

Multiply both sides by \(\displaystyle g^{\frac{1}{2}}\)

Then, divide by \(\displaystyle 1.85u^{\frac{3}{4}}\)

Then, take the fourth power of both sides and you're done.