If f(10)+f(5)=55, what is the value of a

Let the function f be defined by f(x)=5x-2a, where a is a constant.

That's simple.

for x, insert 10 and 5, i.e.:

5 * 10 - 2a + 25 - 2a = 55
75 - 4a = 55
4a = 20
a = 5

(50-2a) + (25-2a)=55

75-4a=55
4a=-20

a=5