hard for you or easy?
Not too bad, here we go:
y= fx gx
y =f x(7x+b)
y= sqrt(7x+b) Can you figure it out now?
one more step substitute in (4,6)
6= sqrt(7(4) + b) Now?
6 = sqrt(28+b)
36 = 28 + b
b = 8
+36919 
