Subtraction in formula

[JasonSimpson]

I am currently attempting to create a field based on a mathematical formula but the formula is getting held up do to the subtraction sign. I know this because I have performed calculations on other parts of the formula with no issue. The formula is shown below and the part I believe is causing issues is in bold. Thanks in advance. 

${gpm_flow} * pow(${flwhyd_static_pressure_psi}-20,0.54) div pow(${flwhyd_static_pressure_psi},0.54 - pow(${reshyd_pressure_psi}, 0.54)

The formula I am attempting is 

[SMauri]

Hi Jason,

try something like this:

${gpm_flow} * (pow(${flwhyd_static_pressure_psi}-20,0.54)/pow(${flwhyd_static_pressure_psi}-${reshyd_pressure_psi}, 0.54))

Cheers

Stefano