{ 2a-5b=-15

you have to solve for one variable in one equation and then substitute…so, here goes:

3a+b=3, subtract 3a from both sides to get ‘b’ alone

b=3-3a

now substitute into the SECOND equation for ‘b’…

2a-5b =-15, since b= 3-3a, you have

2a-[5(3-3a)]=-15

2a-15+15a=-15, combine like terms

17a-15=-15, add 15 to both sides to get ‘a’ alone

17a=0, divide by 17

a=0

take your answer and substitute it into either equation and you have:

3a+b=3

3(0)+b=3

0+b=3

b= 3

note: you cannot combine like terms as you asked from the start because these are both equations themselves…if there was only 1 equal sign then you would be able to combine like terms… 🙂

hope this helped…

b = 3 – 3a

Substitute for b in the second…

2a -5(3-3a) = -15

2a – 15 + 15a = -15

17a = 0

a = 0.