Determine Marginal Revenue Product(MRP) from production function and demand function

A Manufacturer determines that his employees produce total of X unit per day where production function is x = 10t/(√t² +19) where it's total number of employees' demand function is p=90/(x+9). Determine Marginal Revenue Product(MRP) when t=9

[AlexKollie-zr]

Thanks the presentation. However, I didn't understood how you differentiate this function P=900/x+9