$63+21x=-20x-21-x$
Collect like terms$63+21x=-21x-21$
Move the variable to the left-hand side and change its sign$63+21x+21x=-21$
Move the constant to the right-hand side and change its sign$21x+21x=-21-63$
Collect like terms$42x=-21-63$
Calculate the difference$42x=-84$
Divide both sides of the equation by $42$$x=-2$