the answer was 5mx^2-8ax+x^2+3a
let's show your
(5m+1)x^2-8ax+3a
distribute
(5m)(x^2)+(1)(x^2)-8ax+3a
5mx^2+x^2-8ax+3a
answer was 5mx^2-8ax+x^2+3a
hope this helps
