Write the following sets in a simpler form (This can be roster form, interval notation or maybe even a single letter/symbol)

a) $\{x\in\mathbb R\ |\ x^2\leq 49\}$
b) $x\in\mathbb R\ |\ x^2\geq 0\}$
c) $\{x\in\mathbb R\ |\ 2<x<4$ or $3<x\leq 5\}$
d) $\{x\in\mathbb R\ |\ x=10$ or $x=20\}$
e) $\{x\in\mathbb R\ |\ x^2-3x+2=0\}$
f) $\{x\in\mathbb R\ |\ x^2-2x+3=0\}$