Let's say that the variable for Lake Athabasca will be "a". Your equation is 1536 (Lake Louise's elevation) = 7a (7 times the elevation of Lake Athabasca) + 45 (45 m higher, which is added to the 7a).
1536= 7a + 45
Subtract 45 from both sides to get the coefficient and the variable on one side.
1491=7a
Divide both sides by 7 to get the variable alone.
1491/7=7a/7
213=a
The elevation of Lake Athabasca is 213 meters.
