Answer:
a) a1=15 first term
b) d=-8 common difference
Step-by-step explanation:
1)a1+d=7
2)a1+a2+a3+a4=12
a1+a1+d+a1+2d+a1+3d=12
4a1+6d=12
2a1+3d=6
We have the system of equations where a1+d=7 and 2a1+3d=6
3a1+3d=21
2a1+3d=6
Subtract the second equation from the first one
a1=15
d=-8
