How would you handle this? im so mad im about to quit my job!!!!?

okay i work for my mother in law as a receptionist! today this co worker of mine had me doin paperwork for the company! okay that fine, but he got mad because it took for ever... there was a section about percentages that took forever because the totals werent adding up! turns out it was his percetages that were wrong! and he argued with me for over two hours telling me to go back and figure it out! instead of just pulling out a calculator and trying to help!! well then he tells my mother in law that im bad at math(kinda tru!) and that it me forever! so she called me bk in her office and told me i need to brush up on my basic knowledge of things because a couple other people have complained!(about letters, ect!) so my question is if they are not likeing something that i am doing then why cxant they come correct me so i can fix it! instead of going behind my bk!! im 19 and this is my first office job, so im still kinda learning!! what would you do?