ada yg tahu ????? please

f(x)=2x + 5
f(-3)= a
f(b)= -7
2(-3)+5=a
-6+5=a
a= -1
2(b)+5=-7
2b + 5 = -7
2b = -7-5
b= -12: 2
= -6
a+b
= -1+-6
= -7