in regular math: if the function is written as function notation, replace all x's in the expression with your number and solve.
in brainly: click the math button, {it looks like a square root sign with an x inside} press whichever function or process you like and in the text box it should say something like " /[operation]{x}{y} "
in those curly brackets around x and y is where your numbers should go. just get rid of the variable and put it in your number to make the function/process they way you want it to go instead of having those placeholder variables.
