Answer
a) $y^4$
b) $x$
Work Step by Step
a)
$y^4 = y*y*y*y$
$y^5 = y*y*y*y*y$
$y^8 = y*y*y*y*y*y*y*y$
Of 4, 5, and 8, the smallest exponent is 4. Thus, the GCF is $y^4$.
b)
$x=x$
$x^{10} = x*x*x*x*x*x*x*x*x*x$
Of 1 and 10, the smallest exponent is 1. Thus, the GCF is $x^1$, or $x$.