You are using functional notation: f(16)f(x)

Syntactically "f(x)" is equivalent to "the variable f multiplied by the variable x, in parentheses", so a single value (such as 'y') should be used. 

y=-2*sqrt(x)+5

Now, the entry "f(16)f(x)" appears to indicate a need to solve for f(16), given the function f(x)=-2*sqrt(x)+5

So simply replace any instance of 'x' with (16) and simplify the expression.  This is best handled through the desktop Algebrator:

Windows: https://www.softmath.com/tr/alg51win-tr-en.exe

Mac: https://www.softmath.com/tr/alg51mac-tr-en.dmg

The solver simplifies the expression by manually replace every instance of 'x': 

Click to see all the steps