Do you mean this ???
ax^2 + 8x + c = 0 ??? if so
If we have only one solution, the discriminant will = 0 .....so.....
8^2 - 4ac = 0 and a + c = 10 ⇒ c = 10 - a
So....substituting, we have
8^2 - 4a(10 - a) = 0
8^2 - 40a + 4a^2 = 0
4a^2 - 40a + 64 = 0 divide through by 4
a^2 - 10a + 16 = 0 factor
(a - 8) ( a - 2) = 0
The solutions are a = 8 or a = 2
Note that if a = 8, then c = 2 or
If a = 2 then c = 8
But a > c so (a, c) = ( 8, 2 )
Check the ggraph here that only one zero exists at x = -1/2 :
https://www.desmos.com/calculator/idsuikzats