In another universe, temperature is measured differently than here on Earth. One scale is measured in degrees Frankenheit (F) and the other is measured in degrees Crazius (C). Suppose that the two scales are related by a linear equation $C = a F + b$. Use the fact that pure water freezes at -90${}^{\circ}$F and 60${}^{\circ}$C, and boils at 150${}^{\circ}$F and 100${}^{\circ}$C to find $a$ and $b$.

$a =$ and $b =$