Let the function f be defined by f(x)=5x-2a , where a is a constant. If f(10)+f(5)=55, what is the value of a?
How do you do this?
Note that $\displaystyle f\left( 5 \right) = 55 - f\left( {10} \right)$.
But $\displaystyle f\left( 5 \right) = 5\left( 5 \right) - 2a = 25 - 2a$
This means that $\displaystyle 55 - f\left( {10} \right) = 25 - 2a$. Which means that $\displaystyle f\left( {10} \right) = 30 + 2a$.
But $\displaystyle f\left( {10} \right) = 5\left( 10 \right) - 2a = 50 - 2a$.
You can solve for $\displaystyle a$ from the last two equations.
What did you get?