Make a the subject of a + 3 = (2a + 7) / r

Multiply both sides by r: ra+3r = 2a + 7Move a to one side: ra -2a = 7-3rFactorise left hand side: a(r-2) = 7-3rMultiply both sides by (r-2): a= (7-3r) / (r-2)