The formula for determining Peter's yearly salary is:

#y = bm + sc# Where:

#y# is the yearly salary: $26,040 for this problem.

#b# is the base monthly salary: $2,100 for this problem.

#m# is the number of months worked: 12 for this problem (1 year)

#s# is the amount of sales: What we are solving for in this problem.

#c# is the commission rate. 2% for this problem. "Percent" or "%" means "out of 100" or "per 100", Therefore 2% can be written as #2/100#.

Substituting and solving for #s# gives:

#$26040 = ($2100 * 12) + (s * 2/100)#

#$26040 = $25200 + 2/100s#

#$26040 - color(red)($25200) = $25200 - color(red)($25200) + 2/100s#

#$840 = 0 + 2/100s#

#$840 = 2/100s#

#color(red)(100)/color(blue)(2) xx $840 = color(red)(100)/color(blue)(2) xx 2/100s#

#($84000)/color(blue)(2) = cancel(color(red)(100))/cancel(color(blue)(2)) xx color(blue)(cancel(color(black)(2)))/color(red)(cancel(color(black)(100)))s#

#$42000 = s#

#s = $42000#

Peter's yearly sales were #color(red)($42,000)#