It is well known that the element magnesium has three naturally occurring isotopes. mg-24 represents 79.0% of all magnesium atoms, mg-25 accounts for 10.0%, and mg-26 is the remaining 11.0%. if you have 234. atoms of magnesium, how many should be mg-24?

Take your total number of atoms which is 234 atoms. 
then calculate the atoms for Mg-24 and Mg-25 add them together then take that away from the total of 234 to find the answer
234 x 0.79=184.86
234 x 0.10= 23.4
184.86+23.4=208.26
234-208.26=25.74-Final answer

Yes you are correct use the scientific method

the catch is that the top numbers are atomic masses the bottom numbers are the numbers of protons and electrons in an uncharged atom. they are the number of that atoms as you would find them on the periodic table. further, the numbers on the left are used to determine the missing numbers on the right

so here we go

atomic mass

239 + 1 = y + 134 + 3 combine like terms

240 = y + 137             subtract 137 from both terms

240 - 137 = y

103 = y

atomic number

94 = z +54 + 0

94 = z + 54               subtract 54 from both sides.

40 = z

answer