What is the first step in solving this system of equations by elimination? 5a + 3b = –9 2a – 5b = –16

Multiply one of the equations so that both equations share a common complementary coefficient.
       In order to solve using the elimination method, you need to have a matching coefficient that will cancel out a variable when you add the equations together. For the 2 equations given, you have a huge number of choices. I'll just mention a few of them.
   You can multiply the 1st equation by -2/5 to allow cancelling the a term.
   You can multiply the 1st equation by 5/3 to allow cancelling the b term.
   You can multiply the 2nd equation by -2.5 to allow cancelling the a term. You can multiply the 2nd equation by 3/5 to allow cancelling the b term.
   You can even multiply both equations.
For instance, multiply the 1st equation by 5 and the second by 3. And in fact, let's do that. 5a + 3b = –9 2a – 5b = –16 5*(5a + 3b = -9) = 25a + 15b = -45
   3*(2a - 5b = -16) = 6a - 15b = -48
       Then add the equations 25a + 15b = -45
   6a - 15b = -48
   =
   31a = -93
   a = -3
       And then plug in the discovered value of a into one of the original equations and solve for b.