Question 596649
since, in the question there is stated that f(x) = g(x), 
therefore, just equalise the two given equation:
f(x) = -2x
g(x) = 5x - 4


it will look like this;
-2x = 5x -4

group the same term on one side of the eq and numbers on the other side;

-2x -5x = -4
* remember== each time a number@unknown is moved across the equal sign, the mathematical sign(+/-) become mirrored!!

-7x = -4

x = -4/-7

since, negative divide by negative number produce positive number, erase the negative sign...

x = 4/7