Sorry, I'm very rusty with this... I have two equations: T = za and T - mg = -ma Somehow, they get "combined" and to solve for "a" it is: a = (m / (z + m) ) * g can someone please explain the process of how it got to this? I figured out that we substitue for T in the second one to get: za - mg = -ma but I get really lost trying to solve for "a". Please help! b.t.w. sorry, I'm not a math or physics student, I'm trying to learn this stuff myself.