If you were to use the substitution method to solve the following system, choose the new equation after the expression equivalent to x from the second equation is substituted into the first equation.
3x + 2y = −21
x − 3y = 4
a. 3(3y + 4) + 2y = −21
b. 3(−3y + 4) + 2y = −21
c. 3x + 2(3y + 4) = −21
d. 3x + 2(−3y + 4) = −21