How do you solve this problem

I need help with this problem.

As a salesperson at Trading Cards Unlimited, Justin receives a monthly base pay plus commission on all that he sells. If he sells $400 worth of merchandise in one month, he is paid $292. If he sells $900 of merchandise in one month, he is paid $407. Find Justin;s salary when he sells $3,200 worth of merchandise.

You need to find his base pay and his commission. Let the base pay be B and let the commission be x (where x is the fraction of the sales that he gets to keep).

B + 400x = 292
B + 900x = 407
500x = 115
x = 23/100
So he gets 23% commission. Now we can find B using x:
B + 400(23/100) = 292
B + 92 = 292
B = 200

So his monthly base pay is $200.

When he sells $3200, he will get:
200 + 23/100(3200)
= 200 + 736
= $936