How to calculate specific heat with significant figures PLEASE HELP?

I am hopelessly lost in chemistry class, I understand how to use the formula q=m*c*delta T until it comes to the end, in my workbook example this is the problem after you plug int he variables:

q=(5.0x10^3g) * (0.4521 j/gc) * (150.0 C-25.0C)

the answer is 280,000J or 2.8x10^5J

When I multiply that out I do not get the same answer, I get 282,562.5 which is close but not the same, what am I doing wrong? How do significant figures fit into this and how to you multiply a number by a scientific notation, I turned 5.0x10^3 into 5,000 first and then multiplied, is that how your supposed to do it?

THANKS IN ADVANCE :)