Need help with a math problem

Question

a collage student completed some course worth 3 credits and some course worth 4 credits. the student earned a total of 59 credits after completing 18 courses

how many courses worth 3 credits did the student take

Answer 1

3x + 4y = 59

x+y =18

3x+3y =54

y = 5 four credit courses

x = 13 three credit courses