What are the differences among expressions, equations, and functions? provide examples of each.

Question is a general term for any well formed combination of mathematical symbols. it may consist of just one ore more number, variable, operator, function, and their arguments that evaluate to a single answer.
example: x^2-10x+1
equation can be one or two values for the variables used depending on the value equated with the expression. 
for example= x^2 -10x+=0
an equation does not output a value. we can solve the equation and find out the values of the variables. the equation would be valid only for certain values of teh variables. 

function alway output a value and function can have solutions based on the input for the values of the variables.
the values of variable in function are assigned.
graphs of function can intersect the vertical line at multiple point. 
some function can not be graphed 
function are subsets of equations
example of a function: suppose the function f(x)=10x+20, what this means is that this function outputs different values from different input values for variable "x" and  if x=10
we have F(10)= 10.10+20=120
so if we substitute different values into x and get the different outputs, and the important point is that every function outputs a value 
finally, remember that both equations and functions use expression. 

Subtracting is the opposite of adding