notice the tickmarks on the graph, those two sides are equal, and those two angles are also equal, now, the (4x-10) side, is shared by both triangles above and below.
namely the triangles above and below have a [S]ide and then an [A]ngle and then a [S]ide in common, thus they're congruent by SAS.
what does that mean? well, it means that 8x-2 = 3x - 5, and you know the rest then.
though for some reason you put a line through the -, to make it 3x+5, so, dunno what it might be, but those sides are congruent either way.